For the Casserole

• 3–4 medium zucchini, thinly sliced
• 1 small onion, finely chopped
• 2 tablespoons olive oil or butter
• 1 cup shredded cheddar or mozzarella cheese
• ½ cup grated Parmesan cheese
• 2 large eggs
• ½ cup sour cream (or Greek yogurt)
• ½ teaspoon salt
• ½ teaspoon black pepper
• ½ teaspoon dried thyme or Italian seasoning

For the Topping

• ½ cup breadcrumbs (or crushed crackers)
• 2 tablespoons melted butter
• 2 tablespoons grated Parmesan