Creating your Zucchini Gratin with Yellow Squash is simple if you follow these easy steps:

1. Preheat the Oven: Preheat your oven to 375°F (190°C).
2. Prepare the Pan: Grease a 9×13 inch baking dish with olive oil or butter to prevent sticking.
3. Sauté the Onions and Garlic: In a skillet, heat olive oil over medium heat. Add the chopped onion and cook for about 5 minutes until softened. Add the minced garlic and sauté for another minute until fragrant.
4. Layer the Vegetables: Spread half of the sliced zucchini and yellow squash in the prepared baking dish. Sprinkle with half of the sautéed onion and garlic mixture. Season with salt, pepper, thyme, and basil.
5. Add Cheese: Drizzle half of the heavy cream over the vegetables, and then sprinkle half of the Gruyere and Parmesan cheese.
6. Repeat Layers: Repeat the layering process with the remaining vegetables, seasonings, cream, and cheese.
7. Top it Off: Finally, sprinkle breadcrumbs evenly across the top of the gratin for a delicious crunch.
8. Bake: Place in the preheated oven and bake for 30-45 minutes, or until the top is golden brown and the vegetables are tender.
9. Cool and Serve: Once baked, allow the gratin to cool for 5-10 minutes before serving. This helps it set and makes serving easier.

These steps will guide you in creating this incredible dish effortlessly.