1. Mix turkey, beef, onion, salt and pepper in bowl. Divide mixture into 8 equal pieces. Flatten each between sheets of waxed paper into 5-inch circles. Divide blue cheese among 4 of the burgers, leaving 3/4-inch border around edge. Top with remaining burgers, pressing edges to seal.
2. Heat nonstick skillet over medium-low heat. Add burgers; cover and cook 10 minutes, turning over halfway through. Serve on buns.