Preheat your oven to 350°F (175°C). Gather your baking dish, preferably a 9x13 inch.

In a large skillet, heat a tablespoon of olive oil over medium heat. Add the thinly sliced onion, stirring occasionally for about 3 minutes until translucent. Then, add the minced garlic and sauté for another 2 minutes until fragrant and softened.

In your baking dish, create a layer of sliced zucchini and yellow squash, alternating them. Season this layer with fresh thyme, salt, and pepper.

In a mixing bowl, whisk together heavy cream, grated Gruyère cheese, and Parmesan cheese until combined. Pour this mixture over the layered vegetables.

Sprinkle any remaining Gruyère or Parmesan cheese on top. Bake for 40 minutes until the top is golden and bubbly.

Once baked, remove from the oven and allow to rest for 10 minutes before serving.