Preheat the oven to 350 degrees. Spray a 9x13 glass pan with cooking spray, then set to the side.
Add oil to a large skillet on medium heat and allow it to warm. Add diced onion. and saute until the onion is softened, about 3-4 minutes.
Place sausage to the pan and break it up with a spatula into small pieces as it cooks. Cook until sausage is no longer pink, about 10-12 minutes, then drain off any extra grease and set to the side.
In a large bowl mix together half of the tater tots, 1 ½ cups cheese and all of the sausage. (see notes) Pour the mixture into the prepared 9x13 pan.
In a medium bowl add eggs and scramble well with a whisk. Add milk, garlic powder, salt, and pepper to the eggs and mix until combined. Pour the egg mixture over the tater tot mixture in the pan..
Add the remaining tater tots to the top of the breakfast casserole in a single layer. Bake for 45-50 minutes or until the eggs are cooked.
Top the casserole with the remaining ½ cup of shredded cheese. Place the casserole back in the oven uncovered until the cheese is melted for about 5-10 minutes, then enjoy!
