1. Cook bacon in a skillet over medium heat. Remove bacon, reserving drippings in pan. Drain bacon; crumble. Increase heat to medium-high. Add onion to drippings in pan; sauté 5 minutes.
2. Preheat oven to 400°.
3. Combine buttermilk, olive oil, and egg.
4. Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and next 5 ingredients (through pepper) in a bowl, stirring well with a whisk. Make a well in center of flour mixture. Add milk mixture; stir just until moist.
5. Stir in bacon, onion, and cheese. Spoon into 12 muffin cups coated with cooking spray.
6. Bake at 400° for 13 minutes or until a wooden pick inserted in center comes out with moist crumbs clinging. Remove muffins from tins; cool on a wire rack.