Make ahead Trim all of the fat and silverskin from the lamb and cut into 1 1/4-inch cubes. Cut each cube almost through at the center to butterfly and give the meat more surface area for the marinade. Round up all of the odd pieces as well - those can be folded over onto the skewer later.
Make buttermilk by combining milk and vinegar in a small bowl and let stand for 5 minutes.
Mix spices in a gallon-size ziplock bag or large nonreactive bowl (stainless steel, plastic, ceramic, or glass). Stir in buttermilk, lemon juice, and minced garlic until evenly combined. Add meat to marinade and make sure everything is evenly coated. Refrigerate for 2 to 24 hours.
Assembly and Grilling: Soak wood skewers in a pan of water for at least 30 minutes. Heat up the grill or grill pan to be very hot.
Cut red onion and bell peppers into 1-inch pieces. Skewer in the order of lamb, pepper, onion, pepper, lamb until there are 4 lamb chunks on each skewer.
Grill lamb kebabs for 7 or 8 minutes, turning every 90 seconds to cook evenly on all sides.