Preheat oven to 400°F.
Grease bottoms only of muffin pans. Crumble sausage in skillet, add chopped onion. Cook until sausage is done. Stir to keep sausage in small pieces. Drain well.
In a medium bowl, combine baking mix, brown sugar, dry mustard, and red pepper. Stir in sausage mixture. Add milk and cheese. Mix just until blended. Batter will be thick.
Fill muffin cups about 2/3 full, spreading batter to edges of cups.
Bake 20 to 25 minutes or until golden brown. Cool slightly before removing from the pan.