Preheat oven to 350 degrees F. Coat a 9×13 casserole dish with nonstick cooking spray.
In a medium nonstick skillet, combine the sausage, onions, and bell pepper over medium-high heat. Cook, breaking up the sausage into crumbles with a wooden spoon, until browned and no pink remains.
Add in the garlic and cook until fragrant, about 20 seconds. Drain the grease from the sausage mixture and spread it in the casserole dish.
In a mixing bowl, whisk together the eggs, milk, garlic powder, salt, pepper, and dry mustard (if using.) Pour the egg mixture over the sausage mixture.
Sprinkle half of the cheese and all of the tater tots into the casserole dish.
Bake the casserole for 55 minutes. Sprinkle the remaining cheese on the top of the casserole and bake for an additional 5 minutes, or until eggs are cooked through.