Preheat oven to 400 degrees F.
In large saucepan, combine potatoes with salted water to cover by 3 inches. Place over high heat; bring to a boil, and cook until tender when pierced with a knife, about 30 minutes.
When potatoes are cooking, make mint pesto: In food processor, combine mint, pine nuts, cheese, and lemon juice and process until roughly chopped. Add oil and puree until smooth. Transfer to a covered container and refrigerate until needed.
Using dampened kitchen string, tie each lamb chop to hold tail in place, curled into the chop, pinwheel-like. Place chops in roasting pan just large enough to accommodate them in single layer and season with slat and pepper. Drizzle oil evenly over chops.
Place lamb in oven and roast for 10 minutes. Reduce heat to 300 degrees F and conti8nue roasting until thermometer inserted into thickest part away from bone registers 150 degrees F for medium, about 10 minutes longer. Remove lamb to warmed platter, cover loosely with foil, and let rest for 5 minutes.
When potatoes are ready, drain and, when just cool enough to handle, peel, place in bowl, and mash. Return potatoes to saucepan and add butter, cream, mustard, and garlic. Season with salt and pepper, place over medium heat and stir until mixture is heated through. Spoon bed of mashed potatoes on each warmed dinner plate, top with 2 lamb chops, and place a spoonful of mint pesto on the side.