Tender zucchini and yellow squash layered with a creamy, cheesy sauce and topped with golden Parmesan breadcrumbs. A delicious side dish for family dinners, holidays, or potlucks!

Servings: 6
Prep Time: 20 minutes
Bake Time: 35–40 minutes
Total Time: About 1 hour

 Ingredients

Vegetables

• 3 medium zucchini, thinly sliced
• 3 medium yellow squash, thinly sliced
• 1 small yellow onion, thinly sliced
• 2 cloves garlic, minced
• 1 tbsp olive oil
• ½ tsp salt
• ¼ tsp black pepper

 Creamy Cheese Sauce

• 2 tbsp butter
• 2 tbsp all-purpose flour
• 1½ cups (360 ml) whole milk
• ½ cup (120 ml) heavy cream
• ½ cup (50 g) shredded Parmesan cheese
• 1 cup (100 g) shredded sharp cheddar cheese
• ½ tsp garlic powder
• ½ tsp dried thyme
• ¼ tsp paprika
• ¼ tsp black pepper
• ½ tsp salt, or to taste

 Crispy Topping

• ½ cup (50 g) breadcrumbs or panko
• ¼ cup (25 g) grated Parmesan
• 1 tbsp melted butter
• 1 tbsp chopped fresh parsley

 Instructions

1. Preheat the oven

Preheat oven to 375°F (190°C).

Lightly grease a 9 × 13-inch baking dish.

2. Prepare the squash

Slice the zucchini and yellow squash into thin, even rounds.

Arrange them on paper towels and lightly sprinkle with salt. Let them sit for about 10–15 minutes.

Pat them thoroughly dry.

This step is important because zucchini and squash release a lot of water during baking.

3. Cook the onion and garlic

Heat olive oil in a skillet over medium heat.

Add the sliced onion and cook for 4–5 minutes, until softened.

Add garlic and cook for another 30 seconds.

Remove from heat.

4. Make the cheese sauce

Melt butter in a saucepan over medium heat.

Whisk in the flour and cook for about 1 minute.

Gradually whisk in the milk and heavy cream.

Continue cooking for 3–5 minutes, whisking frequently, until slightly thickened.

Reduce the heat to low.

Stir in:

• Parmesan
• Cheddar
• Garlic powder
• Thyme
• Paprika
• Black pepper
• Salt

Stir until the cheese is melted and the sauce is smooth.

5. Layer the vegetables

Arrange a layer of zucchini and yellow squash in the baking dish.

Scatter some sautéed onion and garlic over the top.

Spoon some cheese sauce over the vegetables.

Repeat the layers until all the vegetables and sauce are used.

6. Make the topping

In a small bowl, combine:

• Breadcrumbs
• Parmesan
• Melted butter
• Parsley

Mix until the breadcrumbs are lightly coated.

Sprinkle evenly over the casserole.

7. Bake

Cover loosely with aluminum foil and bake for 20 minutes.

Remove the foil and continue baking for another 15–20 minutes, until the vegetables are tender and the top is golden brown.

8. Rest

Let the au gratin rest for 10 minutes before serving.

This gives the sauce time to settle and makes it easier to serve.

 Tips for the Best Au Gratin

• Slice the squash thinly and evenly.
• Don’t skip drying the vegetables.
• Use freshly grated cheese when possible for a smoother sauce.
• Don’t make the cheese sauce too thin.
• If the top browns too quickly, cover it loosely with foil.
• Let the casserole rest before serving.

 Easy Variations

 Garlic Herb

Add extra garlic and fresh thyme or parsley.

 Spicy

Add a pinch of red pepper flakes or cayenne.

 Extra Cheesy

Add another ½ cup of shredded mozzarella.

 Bacon

Add cooked, crumbled bacon between the vegetable layers.

 Mediterranean

Add a small amount of chopped sun-dried tomatoes and feta.

 Nut & Allergy Information

The recipe contains no peanuts or tree nuts as written.

Potential allergens:

• Milk/dairy: butter, milk, cream, cheddar, Parmesan
• Wheat/gluten: flour and regular breadcrumbs

Gluten-Free

Use gluten-free flour and gluten-free breadcrumbs.

Nut-Free

No nuts are needed.

Dairy-Free

Use dairy-free butter, milk, cream, and cheese alternatives.

Always check packaged ingredients for allergen and cross-contact information if preparing this for someone with a serious allergy.

 Recipe Q&A

Why is my au gratin watery?
Zucchini and yellow squash contain a lot of water. Salting, resting, and thoroughly drying them before baking helps. Also make sure your cheese sauce is properly thickened.

Can I skip salting the vegetables?
Yes, but removing some moisture first gives you a better final texture.

Can I use only zucchini?
Absolutely. Replace the yellow squash with an equal amount of zucchini.

Can I use only yellow squash?
Yes. The same substitution works in reverse.

Can I prepare it ahead?
Yes. Assemble the casserole, cover, and refrigerate for up to about 24 hours. Bake until hot and bubbling.

Can I freeze it?
You can, although squash may become softer after freezing and thawing. For the best texture, enjoy it fresh.

Can I make it without breadcrumbs?
Yes. Simply use Parmesan cheese on top or leave the topping off.

How long do leftovers last?
Refrigerate leftovers in an airtight container and use within 3–4 days.

How do I reheat it?
Reheat in a 350°F (175°C) oven until hot, or use the microwave for individual portions.

 Approximate Nutrition

Per serving, assuming 6 servings:

• Calories: ~300–350 kcal
• Protein: ~13–16 g
• Carbohydrates: ~20–24 g
• Fat: ~20–23 g
• Fiber: ~3–4 g

Nutrition is approximate and depends on the cheese, cream, and breadcrumbs used.

🍽️ What to Serve With It

This au gratin pairs beautifully with:

• Roast chicken
• Baked fish
• Grilled chicken
• Turkey
• Roast beef
• Green salad
• Crusty bread

 Social Media Caption

ZUCCHINI & YELLOW SQUASH AU GRATIN

Tender summer squash baked in a creamy, cheesy sauce and finished with a golden crispy Parmesan topping!

It’s the perfect comforting side dish for a family dinner or holiday table.

Save this recipe and give it a try!