1. Preheat oven to 350°F (175°C) and grease a 9×13-inch casserole dish with butter.

2. In a large skillet, melt butter over medium heat. Add sliced squash and chopped onion; sauté for 8–10 minutes until softened and excess liquid evaporates. Drain any remaining moisture.

3. In a mixing bowl, whisk eggs, milk, mayonnaise (or sour cream), salt, and pepper until smooth.

4. Stir shredded cheddar cheese into the egg mixture, then fold in the sautéed squash and onions.

5. Pour mixture into the prepared casserole dish, spreading evenly.

6. Sprinkle breadcrumbs or crushed crackers over the top, then drizzle with a little melted butter.

7. Bake uncovered for 30–35 minutes until the topping is golden brown and edges are bubbling.

8. Let the casserole rest for 5–10 minutes before serving. Garnish with fresh herbs if desired.