Preheat your oven to 400°F (200°C).

In a sauté pan, heat a drizzle of olive oil over medium heat. Add thinly sliced onion and minced garlic, cooking for about 5 minutes until the onion turns translucent.

In a large mixing bowl, combine the sautéed onion and garlic with thinly sliced zucchini and yellow squash. Pour in the heavy cream, add both Gruyère and Parmesan cheese, along with fresh thyme, salt, and pepper. Stir until the vegetables are evenly coated.

Grease your prepared baking dish with olive oil. Layer the zucchini and yellow squash mixture evenly into the dish.

Cover the baking dish with aluminum foil and bake for 30 minutes.

Remove the foil and return the gratin to the oven. Bake for an additional 10 minutes until the top is golden brown and bubbly.

Let the gratin rest for about 10 minutes before serving.