Preheat the oven to 350 degrees Fahrenheit and spray a 9×13 baking dish with nonstick spray.: The warm, steady heat is essential for even baking, and you should feel the quiet hum of the oven warming as you set the dish inside. Preheating ensures the cornbread begins rising as soon as it goes in, creating a nice crust. If the oven is not fully preheated, the topping may bake too slowly and become dense. A common mistake is skipping spray, which risks sticking; take a moment to coat the pan thoroughly, and the casserole will lift out cleanly.
Cook 1 1/2 pounds lean ground beef in a large skillet over medium-high heat until no longer pink. Add the diced 1/2 cup onion and cook until tender.: As the ground beef browns you will hear gentle sizzling and smell a rich, savory aroma. Browning develops complex flavors through caramelization, which is why this stage matters. When you add the diced onion , it should soften and release a sweet, fragrant scent that melds with the meat. Avoid crowding the pan, because that causes steaming rather than browning, and you will miss those flavorful brown bits. Break the meat into even pieces so it cooks uniformly and drains properly if needed.
Add 1 (15 ounces) can corn, 1 (15 ounces) can chili beans, 1 (10 ounces) can Rotel tomatoes, 1 teaspoon salt, 1/2 teaspoon black pepper, 1/2 teaspoon garlic powder, 2 teaspoons chili powder, and 2 teaspoons cumin. Stir to combine.: When you add the canned ingredients, expect a gentle bubbling and an increase in steam as the mixture simmers, marrying the spices and liquids. The spices will bloom in the hot fat, releasing deeper aromatics that transform a simple skillet into a cohesive filling. Stir thoroughly so the beans and corn are evenly distributed; pockets of unmixed spice can create uneven flavor. Overcooking at this stage can dry the meat, so heat just enough to blend and thicken slightly, about a few minutes, then remove from heat.
Pour the beef mixture into the prepared pan, then top it with 2 cups Colby-Jack cheese.: As you spread the filling in the pan, notice the glossy sheen from the sauces and the visual contrast between the meaty base and the forthcoming golden top. Topping with shredded Colby-Jack cheese creates a molten layer that melts into the filling and forms a creamy barrier beneath the cornbread. Distribute the cheese evenly to avoid dry spots in the finished bake. A common error is piling cheese unevenly, which can cause the cornbread to set differently across the surface.
Combine 2 (8.5 ounce) boxes Jiffy corn muffin mix, 1 (15 ounces) can creamed corn, and 1/2 cup sour cream in a medium bowl and mix until you have a thick batter.: The batter should be thick and slightly lumpy, not runny, and you will notice a sweet, corn-forward aroma. The creamed corn and sour cream keep the mixture moist so the cornbread sets tender over the hot filling. Overmixing can make the top tough, so stir until ingredients are just combined. If the batter seems too thin, a light rest lets it thicken slightly; if it is too thick, a tablespoon of milk can loosen it.
Spoon the mixture evenly over the casserole.: Use a spoon to dot the batter over the cheese layer, then gently spread or nudge the dollops together so they form a mostly continuous surface. As you work, the contrast between the pale batter and the bubbling edge is a good visual cue that you are close to oven-ready. Working too aggressively can push the batter into the filling and cause uneven baking, so be gentle. Aim for an even thickness so the top and filling finish at the same time.
Bake for 35-40 minutes or until the cornbread is golden brown.: While baking you will see the cornbread rise and the edges begin to turn golden, releasing a warm, toasty scent. The cheese underneath will bubble gently, indicating the filling is hot. A toothpick inserted in the center should come out mostly clean with a few moist crumbs but not raw batter. Overbaking will dry the cornbread, while underbaking leaves it doughy, so check visually and by testing for a slight spring to the top. If the edge is browning too quickly, tent loosely with foil for the remaining bake time.