1. Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish to prevent sticking.
2. In a large skillet, heat olive oil or butter over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes. Stir in the garlic and cook for another minute until fragrant.
3. While the onions cook, thinly slice the zucchinis and set them aside. If you prefer, you can lightly salt the zucchini slices and let them sit for 10 minutes to draw out excess moisture, then pat dry. This helps prevent a watery casserole—a tip I learned from Mama Lou herself!
4. In a large mixing bowl, whisk together the eggs, sour cream, mayonnaise, thyme, salt, and pepper until smooth.
5. Add the sautéed onion and garlic mixture to the bowl. Then fold in the shredded cheddar, mozzarella, and Parmesan cheeses, reserving a small handful of cheddar for topping.
6. Gently fold in the sliced zucchini, making sure each piece is coated with the cheesy mixture.
7. Transfer the mixture to the prepared baking dish, spreading it evenly. Sprinkle the reserved cheddar cheese and breadcrumbs evenly over the top for a golden crust.
8. Bake uncovered for 35-40 minutes, or until the casserole is bubbly and the top is a beautiful golden brown. A toothpick inserted into the center should come out clean.
9. Remove from the oven and let it rest for 10 minutes before serving. This pause allows the casserole to set and makes for easier slicing.

One of my favorite moments is pulling this casserole out of the oven and watching Ouma’s eyes light up. It’s always the same look—pure joy and anticipation that makes all the prep worth it. Mama Lou would remind us to savor not just the food but the time spent together around the table.