Place steak and onion in resealable food storage plastic bag. In small bowl, combine lime juice, tequila, oil, garlic, chili powder, cumin and salt; mix well. Pour over steak and onion in bag. Seal bag; turn bag to distribute marinade. Refrigerate 3 hours, turning occasionally.
Heat 12-inch nonstick skillet over high heat until hot. Add steak, onion and all of marinade. Add bell peppers; cook and stir until beef is no longer pink, bell peppers and onion are crisp-tender and most of liquid has evaporated.
Place bun halves, cut side up, on ungreased cookie sheet; broil 4 to 6 inches from heat for 1 to 2 minutes or until toasted. Spoon beef and vegetables onto bottom halves of buns. Top each with 2 pices of cheese; return to broiler for 1 minute to melt cheese. Cover with tops of buns. If desired, top with salsa, sour cream or guacamole.