Combine first 5 ingredients in a medium bowl; make a well in center of mixture. Combine onion, milk, oil, and egg white; add to dry ingredients, stirring just until dry ingredients are moistened.
Spoon batter into 6 miniature (1 3/4-inch) muffin pan cups coated with cooking spray, filling each three-fourths full. Bake at 425° for 17 minutes or until golden. Remove from pan immediately.