Preheat the oven to 375°F (190°C). Grease a 9×13-inch baking dish with butter or cooking spray.
Prepare the zucchini: Slice the zucchinis into thin rounds. You can also spiralize them for a different texture. If you prefer, you can salt the zucchini slices and let them sit for 10 minutes to release excess moisture. Then, gently pat them dry with a paper towel.
Make the creamy mixture: In a large bowl, whisk together the eggs, heavy cream, melted butter, garlic powder, onion powder, salt, and pepper. Add in the mozzarella, cheddar, and Parmesan cheese, saving about 1/4 cup of mozzarella to sprinkle on top later.
Combine zucchini with cheese mixture: Add the sliced zucchini into the bowl with the cheesy mixture. Stir gently to coat the zucchini evenly.
Transfer to baking dish: Pour the zucchini mixture into the prepared baking dish. Spread it out evenly, then sprinkle the reserved mozzarella cheese on top.
Bake the casserole: Place the dish in the oven and bake for about 30-35 minutes, or until the casserole is bubbly, golden, and the zucchini is tender.
Garnish and serve: Once done, remove the casserole from the oven. Let it cool for a few minutes before slicing. Garnish with fresh parsley for added flavor and color.