For the Gratin: Preheat the oven to 350 degrees F.
Slice the potatoes into 1/4-inch slices. Slice the leeks into thin circles. Mix the butter and the garlic together and set aside. Season the cream with salt, white pepper, and nutmeg. Layer the potatoes, the leeks, the garlic butter, and then the cream, alternating until the potatoes are lightly covered with the cream. Bake for 30 minutes.
For the Lamb: Sear the lamb in olive oil in a roasting pan until golden, fat side down first. Remove and allow to rest for 5 minutes. Brush the mustard onto both sides of the seared lamb. Season with salt and pepper. Mix together all of the herbs. Dredge both sides of the lamb in the herbs, pressing lightly so the herbs adhere. In the same roasting pan the lamb was cooked in, add the 20 cloves of garlic and place the lamb on top. Cook in the oven for 15 minutes or until medium-rare. When cooked remove the lamb from the roasting pan and wrap in aluminum foil to rest.
Place the roasting pan on top of the stove over medium heat. Remove the garlic and set aside. Discard all the fat. Add the thyme, white wine, and the stock and reduce by half.
Place half of the roasted garlic in a blender. Add the reduced stock and wine mixture and puree until smooth. Add the tablespoon of butter and pulse to incorporate. Season with salt and pepper.
To Serve: Place a large scoop of the gratin in the center of a plate. Slice the lamb every two chops and place on top of the gratin. Pour the sauce over the lamb and garnish with the remaining roasted garlic and your favorite seasonal vegetables.
Recipe courtesy of The Plaza, New York