In a nonstick skillet, saute onion in butter until tender; set aside. In a bowl, combine cheese and biscuit mix. Stir in milk and egg just until moistened. Fold in ham and onion mixture.
Coat muffin cups with cooking spray or use paper liners; fill three-fourths full. Bake at 425° for 13-15 minutes or until a toothpick inserted near the center comes out clean. Cool for 5 minutes before removing from pan to a wire rack. Serve warm.