In a large skillet over medium-high heat, cook the sausage, potatoes, and onion until the sausage is cooked through and the potatoes are tender, about 10 minutes.
In a 9x13-inch baking dish, layer the sausage mixture, cheese, and soup. Pour the milk over the top.
Bake for 30 minutes, or until heated through and the cheese is melted.
Season with salt and pepper, to taste.
Serve warm.
Interesting Facts
This casserole is a great way to use up leftover potatoes and sausage.
You can easily double this recipe to feed a larger crowd.
If you don't have cream of mushroom soup, you can substitute cream of chicken or cream of celery.