If the dough is a little too dry and crumbly, add more buttermilk, 1 tablespoon at a time.
Preheat oven to 450°.
Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and next 5 ingredients (through pepper) in a medium bowl, stirring with a whisk. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Stir in onions. Add buttermilk; stir gently just until a soft dough forms. Spoon batter evenly into 6 muffin cups coated with cooking spray. Bake at 450° for 12 minutes or until golden.