Cook sausage in 10 in. skillet over medium-high heat, stirring frequently, until no longer pink; drain. Layer sausage, onions, potatoes and cheese in baking dish. Stir remaining ingredients in large bowl with fork or wire whisk until blended. Pour over ingredients in baking dish.
Bake uncovered 35-40 minutes or until knife inserted in center comes out clean. Let stand 5 minutes before serving.