Preheat oven to 350°F.
Brown sausage in a large skillet.
Whisk the egg and milk while the sausage is cooking. Once combined, add half of the cheese.
Drain any fat from sausage, then add it to the egg mixture.
In a 9x11” baking dish, spread the frozen pierogies (no need to thaw) and pour the egg and sausage mixture on top.
Sprinkle with the remaining cheese and bake the casserole for 45 minutes until everything is cooked through.