Rinse the lamb back under cold water and pat dry with paper towels.
Squeeze one lemon into a bowl. Wash the second lemon, pat dry, and cut into thin slices. Peel the garlic cloves and press them. Strip the rosemary leaves from the stems. Mix the lemon juice with 5 tablespoons of oil, the pressed garlic, thyme, and rosemary leaves. Brush the lamb all over with this marinade. Place the lamb and any remaining marinade in a plastic bag, seal, and refrigerate overnight.
Peel, wash, and halve the potatoes. Peel and halve the shallots. Remove the lamb from the refrigerator and pat dry. Heat 2 tablespoons of oil in a large roasting pan over medium-high heat. Season the lamb with salt and pepper. Sear the lamb in the hot oil for 2-3 minutes per side until browned, then remove and set aside.
Place the potatoes and shallots in a roasting pan with 2 tablespoons of oil. Season with salt and pepper. Place in a preheated oven at 200°C (392°F) and roast for 20 minutes.
Reduce oven temperature to 150°C (302°F). Add the lemon slices and seared lamb to the roasting pan with the vegetables. Pour the meat broth into the pan. Return to the oven and braise everything together for 50 minutes, basting with the broth every 10-15 minutes.
Wash and dry the tomatoes. Make a shallow cross-cut at the top of each tomato. Add the tomatoes to the roasting pan 10 minutes before braising is finished (at the 40-minute mark) and return to the oven for the remaining 10 minutes.
Remove the roasting pan from the oven. Remove the lamb from the pan and let rest for 3-5 minutes. Remove the meat from the bones and cut into slices. Divide the potatoes, shallots, and tomatoes among serving plates. Top with lamb slices, drizzle with pan juices, and garnish with fresh rosemary sprigs.
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