I used to follow a typical potato gratin recipe for many years. Then one day, I was out of potatoes. Plus, I was following a more stricter all natural diet and lower carbohydrate diet and I had zucchini on hand. Since this inception of the recipe, I have tried many variations of the recipe. This particular recipe is a fantastic variation. Tomatoes or another vegetable could be substituted for the mushrooms if not preferred. —amrecipes
Preheat oven to 350°F. Spray a 2-quart casserole dish with olive oil cooking spray.
In a small saucepan on medium heat, simmer Parmesan and Asiago cheese, onions, garlic, rosemary, pepper, milk and sour cream for approx. 7 to 8 minutes. Stirring occasionally, until onions are almost translucent.
In casserole dish, layer half zucchini slices, then half mushrooms and bell pepper. Pour in half onion and cream mixture; distributing onion evenly. Layer half slices of mozzarella. Repeat layers ending with a final layer of cheese.
Bake for 30 minutes, until a golden crust has formed on top and the casserole is bubbling. Let sit for 10 minutes. Serve hot or warm.