Combine marinade ingredients in small bowl. Place beef steak and marinade in food-safe plastic bag; turn steak to coat. Close bag securely and marinate in refrigerator 6 hours or as long as overnight, turning occasionally. Remove steak from marinade; discard marinade. Place steak in center of grill; arrange onions and bell peppers around steak. Grill flank steak, uncovered, 17–21 minutes (skirt steak 10–13 minutes) for medium rare (145 degrees) to medium (160 degrees) doneness, turning occasionally. Grill vegetables 13–16 minutes or until crisp-tender, turning occasionally. Meanwhile, combine Avocado Salsa ingredients in medium bowl. Set aside. Carve flank steak lengthwise in half, then crosswise into thin slices. (Carve skirt steak diagonally across the grain into thin slices.) Cut bell peppers into 1/2-inch strips; coarsely chop onions. Place steak slices on tortillas; top with vegetables. Season with salt and pepper, as desired. Serve with Avocado Salsa.