2) Heat a charcoal grill with coals. Spread the coals in one tight layer on the grill.
3) Cut the red onions in quarters and separate each quarter into 3 or 4 sections. Loosely thread 3 or 4 pieces of lamb alternately with sections of onion on skewers. Sprinkle both sides of the lamb with salt and pepper. Next place 4 or 5 cherry tomatoes on skewers, threading them through the stem of the tomato. Rub the tomatoes with olive oil and sprinkle with salt and pepper. First, place the lamb skewers on the hot grill and cook for 10 to 15 minutes, turning 2 or 3 times, until the lamb is medium rare. Approximately 5 minutes before the lamb is done, place the tomato skewers on the grill, turning once, until seared on the outside but still firm inside.
4) For the sauce, bring the chicken stock, olive oil, and lemon juice to a boil in a medium pot. Lower the heat and simmer for 5 minutes, until reduced in half. Add the rosemary, half a teaspoon of salt, and the pepper. Serve the sauce on the side.