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Thin slices of zucchini and yellow squash turn soft and silky under cream, then finish with a bubbling, deeply browned top that crackles when you spoon into it. The vegetables keep their shape just enough to give each bite some structure, while the gruyère and panko add a salty, nutty crust that makes this feel like more than a simple baked side dish.

What makes this gratin work is the balance between moisture and heat. Zucchini and yellow squash release a lot of liquid, so the cream and broth need enough body to coat the layers without turning the dish soupy. Baking it covered first lets the vegetables soften evenly, then uncovering it gives the top time to brown before the center dries out. I also cook the onion down before it goes in, which keeps it sweet instead of sharp.

Below you’ll find the small details that matter most here: how thin to slice the squash, why the breadcrumb topping stays crisp, and the swaps that still give you a proper gratin when you need to work with what you have.

The Gratin Falls Apart When the Vegetables Dump Too Much Water

The biggest mistake with a squash gratin is treating zucchini like a dry vegetable. It isn’t. Once it heats up, it starts giving off moisture fast, and if you pile it into the dish raw with no plan, the cream can thin out and the topping can go soggy before the center is done.

This version handles that problem by building flavor in layers. The onion cooks first so it softens and sweetens, the squash goes in raw but in a tight overlap so it bakes into a neat structure, and the cream-broth mixture gets seasoned before it hits the pan. Covered baking does the gentle work, then the final uncovered stretch gives the top time to brown while the liquid reduces underneath.

What the Gruyère, Cream, and Breadcrumbs Each Bring to the Pan

• Zucchini and yellow squash — Use both if you can. The mix gives you a nicer color contrast and a slightly better texture than using only one squash. Slice them thin and evenly so they cook at the same rate; thick slices stay firm while the rest turns soft.
• Gruyère — This is the backbone of the flavor on top and just under the crust. It melts smoothly and brings that nutty, savory edge a milder cheese can’t match. Swiss works in a pinch, but the flavor will be softer.
• Heavy cream and broth — The cream gives richness, while the broth loosens it just enough to seep between the layers without becoming heavy. Chicken broth adds a little more depth, but vegetable broth keeps the dish fully vegetarian and still tastes balanced.
• Panko and parmesan — Panko stays lighter and crunchier than regular breadcrumbs, which matters because the vegetables underneath are already soft. Parmesan adds salt and sharpness, so don’t overdo the salt earlier if your cheese is very salty.
• Fresh thyme — Dried thyme can work, but use less of it because it reads stronger once baked. Fresh thyme keeps the dish from tasting flat and gives the cream a clean, herbal note.

How to Layer the Squash So the Center Bakes Through

Soften the Onion First

Cook the onion in butter until it turns translucent and sweet, not browned. If it takes on color too fast, lower the heat and give it another minute; raw onion will stay sharp inside the gratin and fight with the cream. The garlic only needs about 30 seconds, just long enough to smell fragrant. Any longer and it can burn before the dish even reaches the oven.

Build Neat Layers in the Dish

Arrange the zucchini and yellow squash in overlapping rows so they look like shingles. That isn’t just for looks; it helps the vegetables cook evenly and gives the gratin enough structure to scoop cleanly. Keep the slices thin and similar in size, because wide variations make the soft pieces collapse before the firmer ones are tender.

Let the Oven Do the Moisture Control

Cover the dish for the first 25 minutes so the vegetables steam gently and start to soften. Then remove the foil and bake until the top turns golden and the edges bubble, which tells you the liquid has reduced enough to support the crust. If it still looks pale and loose in the center, give it a few more minutes uncovered; pulling it too early is how you end up with a runny gratin.

Ways to Bend This Gratin Without Ruining the Texture

Questions I Get Asked About This Recipe

Zucchini Gratin with Yellow Squash

Zucchini gratin with yellow squash layers thin-sliced summer vegetables in a creamy, golden bake. Topped with a bubbling gruyère-and-breadcrumb crust, it’s a classic French squash gratin style side dish.

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Prep Time 20 minutes mins

Cook Time 45 minutes mins

Total Time 1 hour hr 5 minutes mins

Servings: 6 servings

Course: Side Dish

Cuisine: French-American

Calories: 420

Ingredients Equipment Method Notes

Ingredients

  

Vegetables

• 2 medium zucchini thinly sliced
• 2 medium yellow squash thinly sliced
• 1 small onion thinly sliced
• 3 garlic cloves minced

Cream mixture

• 2 tbsp butter
• 1 cup heavy cream
• 0.5 cup chicken or vegetable broth
• 1 tsp fresh thyme
• 0.1 salt to taste
• 0.1 black pepper to taste

Cheese and topping

• 1 cup gruyère or Swiss cheese shredded (divided)
• 0.25 cup parmesan grated
• 0.33 cup panko breadcrumbs
• 2 tbsp butter melted (for topping)

Equipment

• 1 sheet pan

Method

 

Prep and preheat

1. Preheat the oven to 375°F and grease a 9x13 baking dish.

Sauté aromatics

1. Sauté the onion in the butter over medium heat for 4 minutes, until softened.
2. Add the garlic and fresh thyme and cook for 30 seconds, just until fragrant, then remove from heat.

Layer vegetables

1. Layer the zucchini and yellow squash slices in the baking dish in overlapping rows, alternating colors.
2. Scatter the sautéed onion and garlic evenly over the top.

Assemble gratin and bake

1. Whisk the heavy cream, broth, salt, and black pepper, then pour evenly over the vegetables.
2. Sprinkle 3/4 cup of the shredded gruyère over the top.
3. Mix the remaining gruyère, parmesan, and panko with the melted butter, then scatter over everything.
4. Cover with foil and bake for 25 minutes at 375°F, until the vegetables are tender and bubbling around the edges.
5. Uncover and bake 15–20 more minutes at 375°F until golden and bubbling.

Notes

For the creamiest texture, slice zucchini and yellow squash evenly so the layers cook at the same rate. Refrigerate covered up to 3 days; reheat in a 350°F oven until hot and re-bubbling (about 15–20 minutes). Freezing is not recommended because the squash can soften and release extra moisture after thawing. For a dairy-light swap, use half-and-half for the heavy cream (flavor will be milder, and the top may brown slightly less).