Squash Gratin with Mornay Sauce in 7 Easy Steps

Squash Gratin with Mornay Sauce is a rich, creamy, and comforting baked side dish made with tender squash, a silky cheese sauce, and a golden breadcrumb topping. It is perfect for holiday meals, Sunday dinners, or anytime you want an easy vegetable side that feels extra special.

Why You’ll Love This Squash Gratin with Mornay Sauce

• Rich and creamy without being difficult to make
• A delicious way to use fresh squash
• Perfect for holidays, potlucks, and family dinners
• Golden, cheesy topping with tender vegetables underneath
• Easy to prepare ahead and bake later

What Is Mornay Sauce?

Mornay sauce is a classic béchamel sauce with cheese added to it. In this recipe, it creates a creamy, velvety coating for the squash and gives the gratin its rich flavor and luxurious texture.

Ingredients

• 2 tablespoons olive oil
• 1 small onion, finely diced
• 2 cloves garlic, minced
• 2 pounds yellow squash, thinly sliced
• 1 medium zucchini, thinly sliced
• 2 tablespoons unsalted butter
• 2 tablespoons all purpose flour
• 1 1/2 cups whole milk
• 1/4 teaspoon ground nutmeg
• 1 teaspoon salt, divided
• 1/2 teaspoon black pepper
• 1 cup shredded Gruyère or Swiss cheese
• 1/2 cup grated Parmesan cheese, divided
• 1/2 cup breadcrumbs
• 1 teaspoon fresh thyme leaves or 1/2 teaspoon dried thyme
• Butter or cooking spray for greasing the baking dish

How to Make Squash Gratin with Mornay Sauce

1. Preheat the oven: Preheat your oven to 375°F. Grease a 9×13 inch baking dish.
2. Cook the vegetables: Heat the olive oil in a large skillet over medium heat. Add the onion and cook for 3 minutes. Stir in the garlic, yellow squash, zucchini, 1/2 teaspoon salt, and black pepper. Cook for 8 to 10 minutes until the squash softens and some moisture cooks off. Transfer to a bowl.
3. Make the Mornay sauce: In the same skillet, melt the butter over medium heat. Whisk in the flour and cook for 1 minute. Slowly whisk in the milk until smooth. Add nutmeg and the remaining 1/2 teaspoon salt. Simmer for 2 to 3 minutes until thickened.
4. Add the cheese: Remove the skillet from the heat and stir in the Gruyère and 1/4 cup Parmesan until melted and smooth.
5. Assemble the gratin: Fold the cooked squash into the sauce, then transfer everything to the prepared baking dish and spread evenly.
6. Add the topping: In a small bowl, mix the breadcrumbs, thyme, and remaining 1/4 cup Parmesan. Sprinkle evenly over the top.
7. Bake: Bake for 25 to 30 minutes until bubbly and golden brown. Let the gratin rest for 5 to 10 minutes before serving.

Tips for the Best Squash Gratin with Mornay Sauce

• Cook the squash first so the gratin does not turn watery.
• Slice the squash evenly so it cooks at the same rate.
• Use freshly shredded cheese for a smoother Mornay sauce.
• Let the gratin rest before serving so the sauce can settle.
• For extra color, broil for 1 to 2 minutes at the end if needed.

Variations

• Add herbs: Try parsley, chives, or rosemary.
• Make it sharper: Use white cheddar instead of Gruyère.
• Add crunch: Mix a little melted butter into the breadcrumbs.
• Use mixed squash: Combine yellow squash and zucchini for a more colorful dish.

What to Serve with Squash Gratin with Mornay Sauce

This recipe pairs beautifully with roast chicken, grilled steak, pork chops, or holiday mains. It also works well alongside our Watergate salad dessert

How to Store and Reheat

Store leftovers in an airtight container in the refrigerator for up to 4 days. Reheat in a 325°F oven until warmed through, or microwave individual portions. For a crisp topping, reheating in the oven works best. Add an editorial external link here to a trusted food safety source if you want extra authority on storage guidance.

frequently Asked Questions

Can I make squash gratin ahead of time?

Yes. You can assemble the dish a day ahead, cover it, and refrigerate it before baking.

What cheese works best in Mornay sauce?

Gruyère, Swiss, white cheddar, and Parmesan all work very well in a creamy cheese sauce.

Why is my squash gratin watery?

Squash naturally holds a lot of water, so sautéing it before baking is the best way to reduce excess moisture.

Can I freeze it?

You can, but the texture of the squash may soften after thawing. It is best enjoyed fresh or refrigerated for a few days.

Recipe Card

Recipe: Squash Gratin with Mornay Sauce

Prep Time: 20 minutes

Cook Time: 30 minutes

Total Time: 50 minutes

Yield: 6 servings

Category: Side Dish

Method: Baked

Cuisine: American / French Inspired

Nutrition Information

Estimated per serving: 260 calories, 10g protein, 14g carbohydrates, 18g fat, 2g fiber. Nutrition values are approximate and can vary based on ingredients used.