Place potatoes into a medium saucepan, cover with cold water, add salt, bring to the boil and simmer until the potatoes are tender. Drain, return the potatoes to the warm pot, place over low heat and allow the potatoes to steam dry. Mash potatoes thoroughly with a masher or pass through a mouli. Stir through milk and butter and season to taste with salt. Keep warm.
Score the fat on the lamb chops. Place the lamb, fat side down, in a large griddle or frying pan. Cook over medium-high heat for 3-5 minutes or until fat renders out. Turn lamb and cook for 1-2 minutes each side or until cooked to your liking. While lamb is cooking, combine rosemary, garlic, lemon zest and one tablespoon oil in a large bowl. Toss lamb chops in marinade and set aside to rest.
Return pan to high heat. Cook green beans for 2-3 minutes or until tender and lightly charred. Remove from pan. Combine cos, green beans, red onion, mint, olives, feta, lemon juice and remaining one tablespoon oil.
Divide salad between four serving plates, top with lamb chops and drizzle over any remaining marinade.