Whisk together mustard, rosemary, soy sauce, garlic, ginger and oil in a glass bowl. Trim lamb of excess fat; coat with marinade using a pastry brush or fingers. Place on a rack in a roasting pan; cover with plastic wrap. Marinate, refrigerated, 6 hours or overnight.

Preheat oven to 450ºF. Remove lamb from refrigerator, let sit temperature for 1 hour. Place lamb in oven; immediately reduce temperature to 350ºF. Roast for 15 to 20 minutes per pound. To check doneness, insert an instant-read thermometer into thickest part of meat, without touching bone. The temperature should register 135ºF to 140ºF for medium rare (2 hours 15 minutes for 7 1⁄2 pound leg of lamb). Let rest at least 15 minutes before carving.

Carve meat with tip end of lamb bone towards you. Slice away from you and toward 2 o’clock, holding knofe parallel to bone. Slice thin or thick pieces according to your preference. You will have to flip the lamb over as you carve to reach the bottom meat.