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 grid over medium, ash-covered coals; arrange onions and bell peppers around Steak. Grill Flank Steak, covered, 11 to 16 minutes (over medium heat on pre-heated grill, 16 to 21 minutes); Skirt Steak 7 to 12 minutes (8 to 12 minutes on gas grill) for medium rare (145°F) to medium (160°F) doneness, turning occasionally. Grill vegetables 13 to 16 minutes or until crisp-tender, turning occasionally.
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 guacamole, if desired.
Nutrition information per serving: 365 Calories; 99 kcal Calories from fat; 11 g Total Fat; 4 g Saturated Fat; 5 g Monounsaturated Fat; 42 mg Cholesterol; 1154 mg Sodium; 36 g Total Carbohydrate; 3.2 g Dietary Fiber; 29 g Protein; 3.6 mg Iron; 9.1 mg NE Niacin; 0.7 mg Vitamin B6; 1.4 mcg Vitamin B12; 4.8 mg Zinc; 4 mcg Selenium;