Cook sausage and onion in a large skillet over medium to med-high heat, stirring until it crumbles and no longer pink. Drain and cool.
Preheat oven to 375ºF. Lightly spray a mini muffin pan with cooking spray. Set aside.
Combine sausage, baking mix, and shredded cheese in a large bowl. Make a well in the center of the mixture. Stir together the soup and buttermilk; add to sausage mixture, stirring just until dry ingredients are moistened.
Spoon until lightly greased mini muffin tins, filling to tops of cups.