In a large skillet over medium heat, cook the sausage with the diced red pepper and onion. Sauté until the sausage is browned and the vegetables are softened, 7–10 minutes. Drain off any excess grease and let the mixture 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 of the pan with a single layer of frozen tater tots. You likely won’t need the entire bag—just enough to cover the bottom.
Evenly sprinkle the sausage mixture over the tater tots, then top with both cheeses. Pour the egg mixture over the top, making sure it’s evenly distributed.
To make ahead: Cover the dish tightly with foil and refrigerate for up to 18 hours.
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 sit for 5–10 minutes before serving.