Mince the garlic, parsley, cilantro and the intact red onion half. Transfer to a resealable bag. Add the EVOO, vinegar, 2 tsp. salt, 1/4 tsp. pepper and the oregano. Refrigerate 1/4 of the marinade.
Add the lamb to the bag and refrigerate for 2 hours, or overnight. Meanwhile, soak 24 six-inch wooden skewers in water for 1 hour.
Preheat a grill or the broiler to medium. Thread 4 lamb pieces, 1 each green and red bell pepper pieces and 2 onion pieces onto each skewer; season.
Grill or broil the skewers to desired doneness, 5 to 7 minutes. Drizzle the reserved marinade on top.