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

2. In a large skillet, melt 2 tablespoons butter over medium heat. Add sliced squash and diced onion; cook until softened and most moisture has evaporated, about 8–10 minutes. Drain any excess liquid.

3. In a bowl, whisk together eggs, milk, sour cream, salt, and pepper. Stir in 1 cup of shredded cheese.

4. Fold the cooked squash and onions into the egg mixture. Pour into the prepared baking dish and spread evenly.

5. Sprinkle remaining 1/2 cup cheese over the top.

6. In a small bowl, mix crushed crackers with melted butter. Evenly sprinkle the cracker mixture over the casserole.

7. Bake for 30–35 minutes, until the top is golden brown and the center is set. Let rest 5 minutes before serving.