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

2. In a skillet, melt 2 tablespoons butter over medium heat and sauté sliced squash and diced onion until softened, about 6–8 minutes. Drain any excess liquid.

3. In a large bowl, whisk eggs, sour cream, salt, and pepper until smooth.

4. Add sautéed vegetables to the bowl and stir to combine.

5. Fold in 1½ cups shredded cheddar cheese, reserving ½ cup for topping.

6. Stir in the Jiffy corn muffin mix until evenly distributed.

7. Spread the mixture into the prepared casserole dish and smooth the top.

8. Sprinkle reserved cheddar cheese and optional crackers or breadcrumbs evenly over the casserole.

9. Dot the top with remaining 2 tablespoons butter.

10. Bake uncovered for 35–40 minutes, until the top is golden brown and bubbly.

11. Let the casserole rest 5–10 minutes before serving.