"This is a favorite boneless leg of lamb recipe, proven time and time again to be a reliable, fabulous dish. Great for rainy days when outdoor grilling is unappealing. Everything cooks in 1 pan and makes a complete meal. Superb for entertaining. We always serve with multi-grain baguette slices to enjoy the vegetable juices. Just do not plan on any leftovers! Serve with bread or baguette to soak up all the wonderful sauce."
Preheat the oven to 450 degrees F (230 degrees C).
Mix 4 cloves garlic, Dijon mustard, leaves from 2 sprigs of rosemary, vinegar, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a small food processor. Blend into a paste. Crush the remaining garlic.
Trim excess fat from lamb and place in a large roasting pan.
Combine tomatoes, onion, 1/4 cup honey, olive oil, remaining crushed garlic, and remaining salt and pepper in a large bowl. Mix well.
Spread rosemary-mustard paste over the top of the lamb. Spoon tomato mixture around lamb. Tuck remaining rosemary and thyme sprigs evenly up against the leg of lamb. Drizzle with remaining 1/4 cup honey.
Roast in the preheated oven for 20 minutes. Reduce oven temperature to 375 degrees F (175 degrees C). Continue roasting until an instant-read thermometer inserted into the lamb reads at least 130 degrees F (54 degrees C), 1 hour to 1 hour and 15 minutes.
Transfer lamb on a cutting board, cover with aluminum foil, and let rest, 10 to 15 minutes. Discard rosemary and thyme stems. Return the roasting pan to the oven to keep the tomato mixture warm.
Slice and arrange lamb on a plate. Spoon the warm tomato mixture and pan juices on top.