Grease 9×13 baking dish.
In medium skillet, brown sausage; drain.
In large bowl, combine sausage, hash browns, onion, milk, 3 cups of the cheese, salt and eggs. Pour mixture into greased baking dish. Top with remaining 1 cup cheese.
Refrigerate, covered, several hours or overnight.
Heat oven to 350 F.
Bake, uncovered, until knife inserted near center comes out clean (50-55 minutes). Let stand 10 minutes, covered. Cut into squares and serve warm with salsa.