Found on-line at Land O Lakes. Marinating the beef simply seals in the great flavors, both spicy and cool, in these steak fajitas. This requires a 4 hour marinading time, not included in prep/cook time.
Combine all marinade ingredients in large resealable plastic food bag; add flank steak. Tightly seal bag. Turn bag several times to coat steak well. Place into 13x9-inch pan. Refrigerate, turning occasionally, at least 4 hours or overnight.
Heat gas grill to medium or charcoal grill until coals are ash white. Remove steak from marinade; discard marinade. Place steak onto grill. Grill, turning once, until desired doneness (10 to 12 minutes). Place onto cutting board; slice across grain into 1/4-inch slices.
Meanwhile, heat 1 tablespoon oil in 8-inch skillet; add green pepper and onion. Cook over medium-high heat, stirring occasionally, until vegetables are crisply tender (6 to 8 minutes).
To serve, place steak slices and vegetables in center of each tortilla; top with sour cream and guacamole, if desired.