Preheat oven to 400°. In a large skillet, cook sausage and onion over medium heat until meat is no longer pink; drain. Transfer to a greased 13x9-in. baking dish. Sprinkle with cheese. Combine eggs and soup; pour over cheese.
Bake, uncovered, 30-35 minutes or until a knife inserted near the center comes out clean. Let stand 5 minutes before cutting.