Preheat the oven to 350°F.
Spray an 8x8x2-inch baking dish with cooking spray. Place the tater tots in the dish.
In a medium skillet over medium-high heat, cook the sausage and onions, breaking up the sausage into small pieces as it cooks, until the sausage is cooked through and lightly browned and the onions have begun to soften, about 5 minutes. If needed, drain off any excess grease.
Place the sausage and onion mixture in the baking dish in an even layer on top of the tater tots.
In a medium bowl or large measuring cup, whisk together the eggs, evaporated milk, garlic powder, onion powder, dry ground mustard, salt, and pepper until well-combined.
Pour the egg mixture into the baking dish.
Bake for 30 minutes (it will not be done yet).
Remove the dish from the oven and sprinkle the cheese evenly over the top of the casserole.
Return the dish to the oven and continue baking for another 20 to 30 minutes or until the eggs are set (to check, insert a knife into the center of the casserole; if it comes out clean with no wet egg, it is done).
Remove the casserole from the oven and let it rest for 10 minutes before slicing and serving. Sprinkle with chopped fresh parsley, if desired.