1Heat oven to 375°F. Spray 12 regular-size muffin cups with cooking spray.
2In 10-inch skillet, cook beef and onion over medium-high heat 5 to 7 minutes, stirring frequently, until beef is thoroughly cooked; drain. Cool 5 minutes; stir in red pepper flakes, roasted red peppers and feta cheese.
3In medium bowl, stir baking mixture ingredients with whisk or fork until blended. Spoon 1 scant tablespoon baking mixture into each muffin cup. Top with about 1/4 cup burger mixture. Spoon 1 tablespoon baking mixture onto burger mixture in each muffin cup.
4Bake 25 to 30 minutes or until toothpick inserted in center comes out clean and tops are golden brown. Cool 5 minutes. With thin knife, loosen sides of pies from pan; remove from pan and place top sides up on cooling rack. Cool 10 minutes longer, and serve.