I know this seems very strange, but believe me it is truly delicious. No-one can quite detect the secret ingredient but just watch them line up for seconds
Pre heat oven to 220 degrees C.
Place the leg of lamb and the onion quarters into a roasting tin and cook for 45 minutes. Turn the temperature down to 180 degrees C, pour the hot coffee over the meat and continue roasting for a further hour or until the lamb is cooked to your liking. Remove the meat from the roasting tin and keep warm, covered in a tin foil tent.
Strain off the liquid from the roasting tin and reserve, discarding the onion. Sprinkle the flour into the tin, add the butter and whisk together. Gradually whisk in the strained coffee liquid and the hot lamb stock. Cook for 3 - 4 minutes, stirring as it thickens like a gravy. Stir in the red currant jelly and creme fraiche and season to taste with salt and freshly ground black pepper. Serve the lamb with the sauce poured over as a gravy