Combine all marinade ingredients in a large resealable plastic bag. Add the meat and ensure its thoroughly coated. Seal the bag and refrigerate for at least 8 hours, or up to 2 days.
Submerge wooden skewers in cold water and let them soak for 15 minutes.
Preheat a gas grill to medium-high, aiming for a temperature between 450°F and 500°F.
Thread the marinated meat, onions, and bell peppers onto the soaked skewers.
Place the skewers on the grill and cook for 14-15 minutes, turning once halfway through. To check for doneness, either use a meat thermometer (lamb should reach 145°F) or cut into a piece to see if its cooked to your liking. Continue grilling for up to 10 more minutes if needed.