Preheat the oven to 350°F. Grease a 9×13 inch baking dish with cooking spray and set aside.
In a large skillet over medium heat, cook and crumble the breakfast sausage with the diced yellow onion until the sausage is no longer pink and the onion is tender. Drain off any excess grease. (If you don't already have cooked bacon, dice it and add it in to cook with the sausage, then drain off the fat.) 1 pound ground breakfast sausage, 1/2 cup diced yellow onion
Spread the cooked sausage and onion evenly across the bottom of the prepared baking dish. Scatter the crumbled bacon over top. Sprinkle 1½ cups of the cheddar cheese over the meat layer, reserving ½ cup for the top. 6 slices bacon, 2 cups sharp cheddar cheese
Arrange the frozen tater tots in a single even layer over the cheese and meat. 1 bag frozen tater tots
In a large bowl, whisk together the eggs, milk, and sour cream until smooth and no lumps remain. Whisk in the garlic powder, ground mustard, smoked paprika, salt, and black pepper until combined. 8 large eggs, 1 cup whole milk, 1/2 cup sour cream, 1/2 teaspoon garlic powder, 1/2 teaspoon ground mustard, 1/2 teaspoon smoked paprika, 1/2 teaspoon salt, 1/2 teaspoon black pepper
Pour the egg mixture evenly over the tater tots, letting it seep down around the tots. Cover tightly with foil and bake at 350°F for 40 minutes.
Remove the foil, sprinkle the reserved ½ cup cheddar over the top, and return to the oven uncovered for 15 more minutes, until the eggs are fully set, the cheese is melted and bubbly, and the tots are golden brown. If the tater tots aren't crispy enough, place the casserole under the broiler (on low) for 2-3 minutes.
Let the casserole rest for 5 minutes before cutting. Top with sliced green onions and serve warm. 3 green onions