Grease 8" square baking pan.
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 baking pan.
Combine onions, cheese, and sour cream. Place small spoonfuls of sour cream mixture on top of batter.