Preheat your oven to 350°F (175°C).

In a medium skillet, heat 2 tablespoons of olive oil over medium heat. Add one chopped onion and three minced garlic cloves. Sauté for about 5-7 minutes until they become translucent and fragrant.

Thinly slice 2 medium zucchinis and 2 medium yellow squashes. In a greased baking dish, arrange the slices in alternating layers of zucchini and yellow squash.

In a mixing bowl, blend 1 cup of heavy cream, 1 cup of grated Gruyère cheese, and ½ cup of grated Parmesan cheese. Add a tablespoon of fresh thyme, along with salt and pepper to taste. Stir until combined, then pour this creamy mixture over the layered vegetables.

Place the baking dish in the preheated oven and bake for approximately 40 minutes until the top is golden brown and the sauce is bubbling.

Remove the gratin from the oven and let it rest for about 10 minutes before serving.