In a large skillet over medium heat, cook the sausage with the diced red pepper and onion for 7–10 minutes, until browned and softened. Drain excess grease and cool slightly.

In a medium bowl, whisk together the eggs, milk, sour cream, salt, and pepper until well combined.

Spray a 9x13-inch baking dish with cooking spray. Line the bottom with a single layer of frozen tater tots.

Evenly sprinkle the sausage mixture over the tater tots, then top with both cheeses. Pour the egg mixture over the top, ensuring even distribution.

To make ahead: Cover the dish tightly with foil and refrigerate for up to 18 hours before baking.

When ready to bake: Preheat the oven to 375°F. Bake the casserole, covered with foil, for 25 minutes. Remove the foil and continue baking for another 20–25 minutes, until the center is set and the top is lightly golden.

Let the casserole sit for 5–10 minutes before serving to allow it to set.