Alternately thread the lamb, tomatoes, and onions onto long skewers. (If the skewers are wooden, soak in water for 30 minutes before using.)
Drizzle over kebabs, the oil, the lemon juice, garlic, oregano, salt and black pepper. Marinate in the refrigerator for at least 4 and up to 12 hours.
Preheat the grill to medium-high.
Remove the lamb from the marinade and pat dry. Place on the grill and cook until lamb reaches desired temperature, turning occasionally, 8 to 10 minutes for medium-rare.
Remove from the grill and arrange the skewers on 4 plates. Divide the sauce among 4 ramekins and place on the plates with the skewers. Serve with lemon wedges on the side and sprigs of mint.
Yield: 4 servings