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Creamy Gruyere Baked Zucchini Gratin

It’s time to take your average side dishes to the next level with this cheesy baked zucchini gratin! It’s creamy and packed full of zucchini, yellow squash, spinach and gruyere cheese.

Summertime means fresh fruits and vegetables from the garden! It’s the best! But come the end of summer, we are usually zucchini’d out. Anyone else? You can only have sauteed or steam zucchini so many times. That’s why we have the best zucchini bread, chocolate zucchini muffins, and zucchini cupcakes on the blog too. If you want more savory options, this baked zucchini gratin is phenomenal! Ooh, and our Mexican zucchini boats too!

I was watching a little Ina Garten one evening waiting for Cade to get home and I NEVER watch tv so I was totally excited to be soaking in a little inspiration. Ina (like we’re on first name basis) was cranking out a little zucchini and rice dish and I have been making my own low carb version ever since. 

Ingredients for Zucchini Au Gratin

You will need a few veggies, a couple of fresh herbs, two kinds of cheese, and a few other ingredients to make this delicious vegetable side dish. Here is your grocery list:

  • Butter: adds flavor and keeps the veggies from sticking to the pan
  • Olive Oil: cooks all the veggies before baking
  • Zucchini: sliced thin
  • Yellow Squash: sliced thin
  • Shallot: adds flavor, if you don’t have a shallot, a yellow or white onion can be used instead
  • Garlic: adds flavor
  • Spinach: adds texture, nutrients and color
  • Fresh Thyme: adds flavor, freshness
  • Fresh Parsley: adds flavor, freshness
  • Nutmeg: this might be surprising, but trust me…it adds a little pop of flavor
  • Lemon Juice: adds a flavor pop, and cuts through the richness
  • Salt and Black Pepper: adds flavor
  • Eggs: acts as a binder to hold the casserole together
  • Heavy Cream: adds richness
  • Gruyere: adds such great flavor and makes the dish so gooey
  • Parmesan: adds salty flavor and golden crispy topping

The measurements for each ingredient can be found in the recipe card down below.

a photo of a serving of cheesy zucchini squash casserole on a small plate.

How to Make Summer Squash Gratin

You don’t want a side dish that is super complicated, especially for during the summer. This recipe is so easy, which is exactly what you want! You can even make it a day ahead and pop it into the oven right before your dinner. Here are the basic steps:

  1. Preheat the oven.
  2. Place a large cast iron skillet over medium-high heat and add the olive oil.  Sauté the zucchini, squash and shallots.
    • NOTE: If you don’t have a cast iron skillet, butter an oven safe dish and set it aside. Sauté the vegetables in a skillet on the stove top. You will transfer everything to the buttered dish for baking.
  3. Add the garlic.a cast iron skillet with zucchini and yellow squash
  4. Add the spinach and stir until wilted. Then add the thyme, parsley, nutmeg, lemon juice, salt and pepper and toss everything together. Transfer it to the buttered cast iron dish.a cast iron skillet on the stove with zucchini, yellow squash and fresh spinach added to the top being stirred with a wooden spoon.
  5. Whisk the eggs, cream, remaining melted butter and parmesan together. Pour the mixture over the veggies then sprinkle the top with the gruyere.
    • NOTE: If you’re using a separate baking dish, transfer the vegetables to the baking dish before pouring the egg mixture over the top.
  6. Bake and sprinkle with the reserved parmesan. Serve!

These instructions can also be found in the recipe card at the end of this post.

a photo of a cast iron skillet full of thinly sliced zucchini and yellow squash, spinach and topped with golden cheese.

Recommended Tools

You don’t have to have a mandolin to slice your zucchini and squash but it sure makes life easier.

Can I Use Frozen Spinach?

Yes, frozen spinach will work great. I would wring out the moisture from the frozen spinach before adding it.

a photo of a serving spoon scooping out a serving of zucchini squash gratin from a cast iron skillet.

Thanksgiving Side Dish

Zucchini casserole isn’t only for summer either. It is timeless and perfect year round. In fact, every year I grimace a bit over the Thanksgiving side dishes. Do I go slow cooker orange carrots or brussels salad (that is always a must for Christmas at least!) Candied pecan bacon brussels sprouts? This year I KNOW my side dish run down. Ready?

Storing and Reheating

Leftover cheesy squash casserole should be stored in an airtight container in the refrigerator. They will keep for up to 5 days. It unfortunately doesn’t freeze well.

To reheat it, I usually zap a single serving in the microwave, but if I’m reheating a large potion, I will reheat it in the oven at 350 degrees F for 10-15 minutes or until heated through.

Look out Ina Garten! This baked zucchini, squash and spinach medley with creamy gruyere cheese is going to be everyone’s favorite of all the favorite way to use garden zucchinis!

More Summer Side Dishes

Servings: 6

Prep Time: 15 minutes

Cook Time: 20 minutes

Total Time: 35 minutes

  • 4 Tablespoons Unsalted Butter, melted and divided
  • 2 Tablespoons Olive Oil
  • 2 Zucchini, sliced thin (we use a mandolin)
  • 2 Yellow Squash, sliced thin
  • 1 Shallot, sliced thin
  • 4 Cloves Garlic, minced
  • 2 Cups Spinach, packed into the measuring cup or see note
  • 1 Tablespoon Fresh Thyme
  • 1/4 Cup Fresh Parsley, chopped
  • 1/2 teaspoon Ground Nutmeg
  • 2 teaspoons Salt
  • 1 teaspoon Pepper
  • 1 Tablespoon Lemon Juice
  • 4 Large Eggs
  • 3/4 Cup Heavy Cream, whole milk or half and half work if needed
  • 1/3 Cup Parmesan Cheese, freshly grated, divided
  • 3/4 Cup Gruyere Cheese, grated

Prevent your screen from going dark

  • Preheat the oven to 350 degrees F.

  • Heat 2 tablespoons of butter and 2 tablespoons olive oil in a large (12-inch) cast iron skillet (see note) over medium-high heat. Add the zucchini, squash and shallots and sauté for 2 minutes.

    2 Tablespoons Olive Oil, 2 Zucchini, 2 Yellow Squash, 1 Shallot, 4 Tablespoons Unsalted Butter

  • Add the garlic to the pan while stirring with a wooden spoon and cook for one minute.

    4 Cloves Garlic

  • Add the spinach to the pan and stir until wilted and then stir in the thyme, parsley, nutmeg, lemon juice, salt, and pepper and toss well. (Transfer to the buttered baking dish if you aren’t using a cast iron skillet now)

    2 Cups Spinach, 1 Tablespoon Fresh Thyme, 1/4 Cup Fresh Parsley, 1/2 teaspoon Ground Nutmeg, 2 teaspoons Salt, 1 Tablespoon Lemon Juice, 1 teaspoon Pepper

  • In a medium bowl, whisk together the eggs, cream, remaining 2 tablespoons melted butter, and the 1/4 cup Parmesan cheese. Pour the mixture all over the spinach and zucchini, then sprinkle with Gruyere.

    4 Tablespoons Unsalted Butter, 4 Large Eggs, 3/4 Cup Heavy Cream, 1/3 Cup Parmesan Cheese, 3/4 Cup Gruyere Cheese

  • Bake for 20 to 30 minutes, until a knife inserted in the center comes out clean. Sprinkle with reserved parmesan and serve!

    1/3 Cup Parmesan Cheese

If you don’t have a cast iron skillet, butter a 10-12″ baking dish with 2 tablespoons of the melted butter and set aside. You will transfer the vegetables to the baking dish before baking.
Frozen spinach works great if you need to use it. 

Serving: 1CupCalories: 375kcalCarbohydrates: 8gProtein: 14gFat: 33gSaturated Fat: 17gPolyunsaturated Fat: 2gMonounsaturated Fat: 11gTrans Fat: 0.3gCholesterol: 185mgSodium: 1050mgPotassium: 538mgFiber: 2gSugar: 5gVitamin A: 2496IUVitamin C: 33mgCalcium: 317mgIron: 2mg

Author: Sweet Basil

Course: 200+ Easy Side Dish Recipes Every Mom Needs

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a photo of a serving spoon scooping out a serving of zucchini squash gratin from a cast iron skillet.

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