In the small bowl, combine 1 tablespoon of crushed peppercorns, the rosemary leaves, mint leaves, garlic, cider vinegar, and the red wine. Put the lamb into the plastic bag and pour in the marinade. Allow to marinade at least 8 hours or overnight, turning occasionally. Reserve the remaining two tablespoons of peppercorns.
Remove the lamb from the refrigerator at least 30 minutes prior to roasting.
Preheat the oven to 350° F/175° C.
Remove the lamb from the bag, reserving the marinade, and place it in the roasting pan. Tie the lamb in two or three places depending on length. Spread the mustard over the surface of the meat and press the remaining peppercorns into the mustard. Pour the reserved marinade around, not on, the meat. Insert the meat thermometer into the thickest part of the lamb.
Bake until the meat thermometer registers 130° to 135° F, approximately 18 minutes per pound, which will give you a medium-rare roast. If you want medium, cook until 140° F. Remove from the oven and loosely cover the roast with aluminum foil. Allow the meat to rest about 15 minutes before slicing. Remove the kitchen twine and slice into 1/2-inch pieces. Serve immediately.