Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper or use a roasting pan.
Prepare the herb crust: In a small bowl, mix olive oil, minced garlic, rosemary, thyme, Dijon mustard, bread crumbs, salt, pepper, and lemon zest.
Pat the lamb dry with paper towels and rub the herb mixture all over. Let it rest at room temperature for 20–30 minutes.
Roast the lamb: Place the lamb in a roasting pan and pour broth around it. Roast for 20 minutes at 400°F, then reduce heat to 350°F (175°C) and continue roasting for:
Medium-rare: 15–20 minutes per pound (internal temp 130°F)
Medium: 20–25 minutes per pound (internal temp 140°F)
Rest and serve: Let the lamb rest for 10–15 minutes before slicing. Serve warm.