In a skillet, cook the sausage over medium heat until browned. Drain excess fat.
In a large bowl, combine the cooked sausage, potatoes, cheese, and onion.
In a small saucepan, melt the butter over medium heat. Stir in the milk, salt, and pepper. Pour over the sausage mixture and stir to combine.
Transfer the mixture to a greased baking dish. Cover with foil and bake for 45 minutes. Remove the foil and bake for an additional 15 minutes, or until the potatoes are tender and the top is golden brown.
Let the casserole cool for a few minutes before serving.
Notes:
This casserole can be made ahead and refrigerated. Just add an extra 10-15 minutes to the baking time if baking from cold.