Place onions and lamb in roasting tin and cook for 45 minutes.
Reduce oven temperature to 180 degrees, pour the hot coffee over the meat and roast for another hour or until cooked to your liking. Remove the meat from the tray and store in foil for resting.
Strain off the liquid from the roasting tin and reserve, discarding the onion. Sprinkle in the flour, add the butter and whisk together. Gradually whisk in the strained coffee liquid and stock until it thickens like a gravy. Stir in the red currant jelly and creme fraiche to taste with the salt and pepper.
Serve with the sauce as a gravy with vegetables of your choosing.