Preparation

Coat and roast the lamb

• Pulse the pine nuts and potato chips in a food processor until the size of breadcrumbs. Transfer to a medium bowl, and add the garlic, rosemary, 1/4 tsp. salt, and 1/2 tsp. pepper.
• In a small bowl, combine the mayonnaise and mustard. Coat the meaty side of the lamb with the mayonnaise mixture, then press the pine nut mixture into this coating. (At this point, you can refrigerate the lamb for a few hours. Let sit at room temperature while the oven heats.)
• Position a rack in the center of the oven, and heat the oven to 400°F.
• Transfer the lamb to a roasting pan or broiler pan, coated side up, and roast, shielding the crust with aluminum foil if it browns too quickly, until medium rare (125°F), 25 to 35 minutes. Transfer to a cutting board, tent loosely with foil, and let rest for 10 minutes before slicing.

做酱

• In a small saucepan over medium heat, combine the port, demi-glace, and 1 cup water. Bring to a boil, and then reduce to 2/3 cup, about 10 minutes. When reduced, lower to a simmer, whisk in the butter, and season to taste with salt and pepper.
• Slice the lamb into chops, and serve with the sauce.