First Step: PREPARE THE ZUCCHINI

1. Wash and Slice the Zucchini: Start by washing the zucchinis thoroughly. Slice them into thin rounds, about 1/4-inch thick. If the zucchini is particularly large, you may want to remove the seeds, but this is optional.
2. Salt the Zucchini (Optional): To remove excess moisture from the zucchini and prevent the casserole from becoming too watery, sprinkle the sliced zucchini with salt. Let them sit for about 10 minutes, then blot with paper towels to absorb the liquid. This step is optional but recommended if your zucchinis are very watery.

Second Step: SAUTÉ THE ONION AND GARLIC

1. Heat Olive Oil: In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until soft and translucent, about 4-5 minutes.
2. Add Garlic: Add the minced garlic to the pan and sauté for another 30 seconds, until fragrant. Be careful not to burn the garlic.
3. Add Zucchini: Add the zucchini slices to the skillet and cook for 5-7 minutes, stirring occasionally, until the zucchini is tender but still slightly crisp. You want the zucchini to soften but not become mushy.
4. Season: Season the zucchini with salt and pepper to taste. Remove from heat and set aside.

Third Step: MAKE THE CASSEROLE BASE

1. Preheat the Oven: Preheat your oven to 375°F (190°C). Grease a 9×13-inch casserole dish with a bit of butter or cooking spray to prevent sticking.
2. Prepare the Egg Mixture: In a large mixing bowl, whisk together the eggs, sour cream, and half of the grated Parmesan cheese. Add in the shredded mozzarella and cheddar cheeses, mixing well. This mixture will form the base of your casserole and help bind everything together.
3. Combine the Zucchini and Cheese Mixture: Add the sautéed zucchini, onion, and garlic to the egg and cheese mixture. Stir until all the ingredients are well combined. Taste and adjust the seasoning with more salt and pepper if necessary.

Fourth Step: ASSEMBLE AND TOP THE CASSEROLE

1. Transfer to the Casserole Dish: Pour the zucchini mixture into the prepared casserole dish, spreading it out evenly. Use a spatula to press it down slightly so that everything is well packed.
2. Add the Breadcrumb Topping: In a small bowl, combine the remaining grated Parmesan cheese and the breadcrumbs. Sprinkle the breadcrumb mixture evenly over the top of the casserole. This will create a crispy, golden topping when baked.
3. Optional Add-Ins: If you’re adding any optional ingredients such as cooked ground meat or chopped bell pepper, sprinkle them evenly over the top before baking.

Fifth Step: BAKE AND SERVE

1. Bake the Casserole: Place the casserole in the preheated oven and bake for 30-40 minutes, or until the top is golden brown and crispy, and the casserole is set in the center. The cheese should be bubbly and slightly browned at the edges.
2. Cool Slightly Before Serving: Once the casserole is done baking, let it cool for about 5-10 minutes before serving. This allows the casserole to firm up and makes it easier to slice.
3. Garnish and Serve: Garnish the casserole with fresh herbs, such as chopped parsley or basil, for a burst of color and freshness. Serve the casserole hot, either as a main dish or a side.