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

2. If using fresh squash, wash and slice into ¼-inch thick rounds. If using frozen squash, thaw completely and drain excess moisture.

3. In a large skillet, melt 2 tablespoons butter over medium heat. Add squash and diced onion; sauté until tender, about 7–8 minutes. Drain any excess liquid.

4. In a large bowl, combine sautéed squash mixture with eggs, sour cream (or mayonnaise), shredded cheddar cheese, salt, pepper, and garlic powder. Stir until well blended.

5. Transfer the mixture into the prepared casserole dish, spreading it evenly.

6. Melt remaining 2 tablespoons butter and toss with bread crumbs or crushed crackers. Sprinkle evenly over the squash mixture.

7. Bake uncovered for 30–35 minutes, until the filling is bubbly and the topping is golden brown. For extra crispiness, broil for 1–2 minutes, watching closely.

8. Let rest for 5 minutes before serving. Garnish with fresh parsley or thyme if desired.