How to Make It
Preheat the oven to 450°. Season the lamb with salt and pepper and put it in a medium roasting pan. Scatter the onions, garlic and carrots around the meat and roast for 15 minutes. Reduce the oven temperature to 325° and roast the lamb until an instant-read thermometer inserted in the thickest part of the meat registers 120°, about 1 1/2 hours. Leave the oven on.
Using a pastry brush or spoon, spread the mustard all over the meat. In a small bowl, combine the minced rosemary with the thyme and 1 1/2 teaspoons each of salt and pepper. Pat the herb mixture all over the lamb and roast for 10 minutes longer, or until an instant-read thermometer inserted in the thickest part of the meat registers 130°. Transfer the lamb to a cutting board and let rest for 20 minutes. Transfer the vegetables to a bowl and keep warm.
Meanwhile, heat a medium skillet until very hot. Sprinkle the sugar in the skillet and cook over moderate heat until caramelized, about 20 seconds. Add the verjus and cook for 10 seconds. Add the Syrah and cook, shaking the pan frequently, until the sugar dissolves and the wine is reduced by half, about 10 minutes. Stir in the demiglace, stock and rosemary sprig and simmer over moderate heat until reduced to 1 cup, about 15 minutes. Add the dried cherries and simmer over low heat just until softened, about 5 minutes. Season lightly with salt and pepper. Discard the rosemary sprig and transfer the sauce to a warmed gravy boat.
Discard the kitchen string and slice the roast thickly. Transfer to a platter, arrange the vegetables around the lamb and serve with the dried-cherry sauce.
Demiglace is available at specialty food stores and butcher shops.