While the lamb is cooking, pull out a large stockpot. Scrub the potatoes, drop into the pot and add enough cool water to cover by 1 inch (2.5 cm). Bring to a rolling boil over high heat, turn the heat to medium-low, and cover. Simmer the potatoes until completely tender, about 30 minutes.☞ TESTER TIP: If you’re pressed for time, you can peel and cube the potatoes before boiling, then proceed with mashing them while warm.