Using slicer/shredder disc, shredding side up, shred cheddar and set aside.
Using S-blade, place onion in work bowl and process until finely chopped. Place chopped onion and butter in a skillet and cook over medium high heat until onions are soft. Set aside and cool. Do not wash work bowl.
Using S-blade, place egg, milk, muffin mix, and corn into work bowl. Pulse until JUST mixed. Pour batter into greased 8-inch square baking pan.
Combine onions, cheese, and sour cream. Place small spoonfuls of sour cream mixture on top of batter. Bake at 425°F for 30 to 35 minutes.
Note: Designed for full-size food processors with a minimum of 6 cup capacity.