A creamy, cheesy vegetable casserole with tender zucchini and yellow squash, topped with golden Parmesan and baked until bubbly and beautifully browned.
Ingredients
For the vegetables
2 medium zucchini, thinly sliced
2 medium yellow squash, thinly sliced
1 tablespoon olive oil
2 cloves garlic, minced
½ teaspoon salt
¼ teaspoon black pepper
½ teaspoon Italian seasoning
For the creamy cheese sauce
2 tablespoons butter
2 tablespoons all-purpose flour
1 cup whole milk
½ cup heavy cream
½ cup shredded sharp cheddar cheese
½ cup shredded mozzarella cheese
¼ cup grated Parmesan cheese
½ teaspoon garlic powder
¼ teaspoon onion powder
¼ teaspoon black pepper
¼ teaspoon salt, or to taste
Pinch of nutmeg, optional
For the topping
¼ cup grated Parmesan cheese
¼ cup shredded mozzarella cheese
2 tablespoons breadcrumbs, optional
1 tablespoon chopped fresh parsley
Instructions
Preheat the oven:
Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish.
Prepare the squash:
Wash and dry the zucchini and yellow squash. Slice them into thin, even rounds. Avoid slicing them too thick because they will release moisture during baking.
Remove excess moisture:
Arrange the squash slices on paper towels and lightly sprinkle with salt. Let them sit for about 10–15 minutes, then pat them thoroughly dry. This helps prevent a watery casserole.
Season the vegetables:
Place the zucchini and yellow squash in a large bowl. Add olive oil, minced garlic, Italian seasoning, black pepper, and a small amount of salt. Toss gently.
Make the cheese sauce:
Melt the butter in a saucepan over medium heat. Whisk in the flour and cook for about 1 minute.
Slowly pour in the milk and heavy cream while whisking continuously. Cook for 3–5 minutes, until the sauce thickens.
Reduce the heat to low. Stir in cheddar, mozzarella, and Parmesan until melted and smooth.
Add garlic powder, onion powder, black pepper, salt, and a tiny pinch of nutmeg if desired. Taste and adjust seasoning.
Layer the casserole:
Arrange half of the zucchini and yellow squash in the prepared baking dish.
Spoon or pour half of the cheese sauce over the vegetables.
Add the remaining squash in an even layer, followed by the remaining cheese sauce.
Add the topping:
Sprinkle Parmesan and mozzarella evenly over the top. If using breadcrumbs, sprinkle them over the cheese.
Bake:
Cover loosely with aluminum foil and bake for 25 minutes.
Remove the foil and continue baking for another 15–20 minutes, until the vegetables are tender and the top is golden brown and bubbling.
For an extra golden top, switch to broil for 1–2 minutes, watching carefully so it doesn’t burn.
Remove from the oven and let the casserole rest for 10 minutes before serving.
Garnish with fresh parsley and serve warm.
Nutrition Information
Approximate per serving — 8 servings:
Calories: 225
Protein: 10 g
Carbohydrates: 10 g
Fiber: 2 g
Net Carbs: 8 g
Fat: 17 g
Saturated Fat: 9 g
Cholesterol: 45 mg
Sodium: 390 mg
Sugar: 4 g
Nutrition will vary depending on the exact cheese, milk, cream, butter, and breadcrumbs used.
Nut Information
Tree nuts: None in the standard recipe.
Peanuts: None.
Coconut: None.
Nutmeg: Although its name contains “nut,” nutmeg is a seed, not a tree nut. It is optional.
Gluten: The standard recipe contains gluten because of the all-purpose flour and optional breadcrumbs. For gluten-free preparation, use gluten-free flour and gluten-free breadcrumbs.
Dairy: Contains milk, butter, cream, cheddar, mozzarella, and Parmesan.
Eggs: None.
Soy: Generally none, but check packaged cheese and breadcrumbs for additives if allergies are a concern.
Helpful Tips
Don’t skip drying the squash. Zucchini and yellow squash contain a lot of water, and drying them helps create a thicker gratin.
Slice the vegetables approximately ⅛–¼ inch thick for even cooking.
Freshly grated cheese usually melts more smoothly than pre-shredded cheese.
For extra flavor, add ½ cup sautéed mushrooms or a little caramelized onion.
For a lower-carb version, simply omit the breadcrumbs.
Letting the gratin rest before serving allows the creamy sauce to thicken.
Frequently Asked Questions
Q: Can I make this ahead of time?
Yes. Assemble the casserole, cover, and refrigerate for up to 24 hours. Add about 5–10 minutes to the baking time if baking directly from the refrigerator.
Q: Why is my squash gratin watery?
The most common reason is excess moisture from the squash. Salt, rest, and thoroughly pat the slices dry before assembling.
Q: Can I use only zucchini?
Absolutely. You can replace the yellow squash with an equal amount of zucchini.
Q: Can I use only yellow squash?
Yes. Yellow squash works equally well.
Q: Can I freeze it?
It can be frozen, but the texture may become softer after thawing because squash contains a lot of water. For the best texture, enjoy it fresh.
Q: Can I make it without flour?
Yes. Use a low-carb thickener such as a small amount of cream cheese or an appropriate keto-friendly thickening method.
Q: What cheese works best?
A combination of sharp cheddar, mozzarella, and Parmesan gives excellent flavor, creaminess, and browning.
Q: What should I serve with zucchini and yellow squash au gratin?
It pairs especially well with roasted chicken, grilled fish, baked salmon, turkey, steak, or a fresh green salad.
Q: Can I make this vegetarian?
Yes. The recipe is naturally vegetarian as written; just check that your Parmesan cheese is suitable for your dietary preference.
Q: How should I store leftovers?
Refrigerate leftovers in an airtight container for up to 3–4 days. Reheat in the oven or microwave until hot.
With years of experience in restaurant research and local SEO, Sara creates accurate, user-friendly guides that work for both people and search engines. Her goal is to help users go from search results to their nearest restaurant or menu choice with as few clicks as possible.
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