Dry leg of lamb with paper towels. Place the leg of lamb in a large roasting pan, fat side up. Combine the mustard, 1 tablespoon chopped garlic (4 cloves), rosemary, balsamic vinegar, 1 tablespoon salt, and 1/2 teaspoon pepper in a mini food processor and pulse until the garlic and rosemary are minced. Spread the mixture evenly over the lamb.
Place tomatoes, olives (if using), olive oil, 1/4 cup of honey, onion, remaining garlic, 2 tablespoons salt, and 2 teaspoons pepper in a bowl and toss well. Pour the tomato mixture around the lamb and tuck in the thyme and rosemary sprigs. Drizzle the lamb with the remaining 1/4 cup of honey.
Roast for 20 minutes. Turn the heat down to 350°F and roast for another 60 to 75 minutes, or until a meat thermometer registers 130 to 135°F for medium-rare. Place the lamb on a cutting board, cover with aluminum foil, and allow to rest for 15 minutes. Discard the herb stems and return the tomatoes to the oven to keep warm. Slice the lamb, arrange on a platter, sprinkle with salt and pepper, and serve with the tomatoes and pan juices spooned over.