About this recipe:This recipe is from the late 19th century. My grandmother brought it with her to the U.S. when she emigrated with her family from London, England. It has been passed down through our family, and is our signature family dinner. It is unusual in that the only spices used are salt and pepper. No mint jelly, please!
Rub the vinegar over lamb and allow to rest for 30 minutes.
Preheat the oven to 170 C / Gas 3.
Season the lamb with salt and pepper, and set on a rack placed inside a roasting pan. Spread the shortening liberally over the lamb.
Roast in the preheated for 3 to 4 hours until the desired level of cook is achieved. The amount of fat around the lamb and marbling throughout will determine the cooking time (see footnote).
An hour before the lamb is due to finish cooking, bring a pan of water to the boil; add the potatoes and cook until almost done, but still slightly firm in the centre. Remove the lamb from the oven then spread out the potatoes under the lamb and roast for the final hour of cooking until soft and brown.
Warm the milk in a saucepan over a medium heat then add the onions and cook until soft. Add half of the butter and the cornflour dissolved in a little water to thicken the sauce. Set aside and keep warm.
Bring a separate saucepan of water to the boil then add the swede and cook until tender, about 12 minutes. Drain then add the sugar and remaining butter and mash. Set aside and keep warm.
Bring a small saucepan of water to the boil then add the peas and cook until tender, about 6 to 8 minutes. Drain. Cook peas and drain. Set aside and keep warm.
Remove the lamb from the oven and allow to rest for 10 to 12 minutes. Carve the lamb and serve with the potatoes and vegetables and onion sauce on the side, or over the meat, as desired.
Roasting the lamb on a stand allows one to spread the potatoes under the lamb so they become soft and flavourful from the drippings; the secret to the unique aspect of this recipe. This is lamb in its purest taste. I'm curious if any readers have prepared lamb in this way without spices, except for salt and pepper.
Shortening is a popular ingredient used in American baking. Although we don't have an exact match here in the UK, you can experiment with vegetable shortening such as Trex which is available in supermarkets. It is also possible to have good success with margarine as a substitute.
To take the guesswork out of the cooking, you can also use a cooking thermometer. To achieve a rare cook, the internal temperature should reach 42 degrees C. For medium-rare 58 degrees C and for medium-well 68 degrees C.