In a large skillet, cook sausage and onion until thoroughly cooked. Drain.
In an 8x8x2-inch (2-quart square) baking dish, layer potatoes, half of the cheese, sausage-onion mixture, and remaining cheese.
In a bowl, combine eggs, milk, and pepper; pour over cheese.
Bake, covered, in a 350 degree F oven for 50 to 55 minutes or until a knife inserted near center comes out clean. Transfer to a wire rack. Let stand for 10 minutes.
Cut into squares. Pass the salsa. Makes 6 servings.
Assemble the casserole as above. Cover and chill in refrigerator for up to 24 hours. Bake as directed.