HEAT oven to 400ºF.
COMBINE first 6 ingredients in large bowl. Whisk egg, buttermilk and oil in medium bowl until blended; stir in cheese, bacon and onions. Add egg mixture to dry ingredients; stir just until moistened.
SPOON into 12 muffin cups sprayed with cooking spray.
BAKE 18 to 20 min. or until toothpick inserted in centres comes out clean. Cool in pan 10 min. before removing from pan to serve.