1. Preheat oven to 350°F (175°C). Grease a 9×13-inch baking dish with cooking spray or butter.

2. Heat oil in a large skillet over medium. Sauté squash, onion, bell pepper, and jalapeño with a pinch of salt until just tender, 7–9 minutes; drain off excess liquid.

3. In a small saucepan or bowl, whisk sour cream with garlic powder, paprika, black pepper, and remaining salt until smooth and warmed.

4. In a mixing bowl, combine sautéed vegetables, creamy mixture, roasted corn (if using), 3/4 cup cheddar, and pepper jack (if using).

5. Spread mixture in the baking dish. Top with remaining cheddar, then sprinkle breadcrumbs evenly; mist with oil or dot with butter.

6. Bake 25–30 minutes until bubbling at the edges and the top is golden brown.

7. Rest 5–10 minutes before serving so the casserole sets for cleaner scoops.

8. Make-ahead: Assemble through topping, cover, and refrigerate up to 24 hours; bake 30–35 minutes from chilled. Store leftovers 3–4 days or freeze up to 2 months.