Preparation: Preheat your oven to 325°F (160°C). Remove the lamb from the fridge 30 minutes before cooking to bring it to room temperature. Pat it thoroughly dry with paper towels.
The Master Glaze: In a small bowl, whisk together the honey, Dijon, whole grain mustard, minced garlic, minced rosemary, salt, and pepper until you have a thick, spreadable paste.
Build the Bed: In a large cast-iron skillet or heavy roasting pan, arrange the baby potatoes, carrots, and onion wedges in an even layer.
Glaze the Lamb: Place the lamb shoulder directly on top of the vegetables. Rub the honey-mustard glaze all over the top and sides of the meat, pressing it in to ensure a thick crust forms.
Slow Roast: Cover the pan loosely with aluminum foil. Bake for 2 hours.
Caramelize: Remove the foil and baste the lamb and vegetables with the savory pan juices. Increase the oven temp to 375°F (190°C) and bake for another 30–45 minutes until the crust is a deep, glistening mahogany and the meat is tender.
The Rest: Transfer the lamb to a warm plate and tent with foil. Let it rest for 20 minutes. This allows the juices to redistribute for a perfectly succulent roast.
Serve: Carve into thick, juicy slices and serve with the roasted vegetables and a drizzle of the remaining pan drippings.