In a medium skillet, heat oil over medium heat. Add the 1/4 cup onion and the garlic; cook and stir until onion is tender. Transfer onion mixture to a medium bowl; cool to room temperature. Stir in lime juice, orange juice, cumin, and oregano.
Trim fat from meat. Place meat in a re-sealable plastic bag set in a shallow dish. Pour marinade over meat. Seal bag; turn to coat meat. Marinate in the refrigerator for 8 to 24 hours, turning bag occasionally. Drain meat, reserving marinade. Brush sweet pepper and onions with some of the marinade; discard the remaining marinade. Stack tortillas and wrap in foil; set aside.
For a charcoal grill, arrange hot coals around a drip pan in a grill with a cover. Test for medium-hot heat above pan. Place meat on the grill rack over pan. Place peppers and onions on the grill directly over coals. Cover and grill for 10 to 12 minutes or until vegetables are crisp-tender, turning vegetables occasionally. Remove vegetables; cover to keep warm. Grill meat for 20 to 25 minutes more or until a meat thermometer registers 155 degrees F. Add tortillas to the grill during the last 10 minutes of grilling or until heated through, turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium-high. Adjust for indirect cooking. Cover and grill as above.)
Remove meat from grill. Cover with foil and let stand for 10 minutes. (The meat's temperature will rise 5 degrees F during standing.) While the meat stands, cut peppers and onions into thin wedges or strips. Slice meat. Serve meat and vegetables on warm tortillas with Orange-Avocado Salsa, sour cream, and, if desired, lime wedges. Garnish with cilantro leaves. Makes 4 servings.
In a medium bowl, combine the oranges, avocado, red onion, cilantro, lime juice, bottled hot pepper sauce, and salt. If desired, cover and chill for up to 4 hours.