Preheat oven to 375°.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour and the next 5 ingredients (through pepper) in a food processor; pulse 3 times to combine. Add 5 tablespoons cheese and butter; pulse 5 times or until mixture resembles coarse crumbs. Spoon mixture into a medium bowl. Combine buttermilk, onions, garlic, and egg; stir with a whisk. Add to flour mixture, stirring just until moist. Spoon batter into 12 muffin cups coated with cooking spray. Sprinkle evenly with remaining 3 tablespoons cheese. Bake at 375° for 18 minutes or until a wooden pick inserted in center comes out clean. Cool for 5 minutes in pan on a wire rack; remove from pan. Cool completely on a wire rack.