In a large skillet, cook beef, onion and garlic over medium heat 6-8 minutes or until beef is no longer pink and onion is tender, breaking up beef into crumbles; drain. Stir in chiles, salsa and cumin.
Spoon 1/2 cup beef mixture across center of each tortilla; top with 2 tablespoons cheese. Fold bottom and sides of tortilla over filling and roll up.
Place chimichangas on grill rack, seam side down. Grill, covered, over medium-low heat 10-12 minutes or until crisp and browned, turning once. If desired, serve with sour cream and guacamole.