To make the marinade: Cut the half onion into four pieces and stud each one with a clove. Combine in a large bowl with the rest of the marinade ingredients. Add the venison, toss well and leave overnight in the fridge to marinate.
Lift the venison out of the marinade (reserving the marinade), drain and pat dry with paper towels. Season the flour and use to coat the venison (the cleanest way to do this is to put the flour and venison in a plastic bag and toss well).
Preheat the oven to 160 degrees Celsius (315 degrees Fahrenheit/Gas 2-3). Heat the oil and clarified butter in a large casserole dish, brown the shallots and then remove from the dish. Brown the venison in the oil and butter, then remove from casserole.
Strain the marinade liquid through a sieve into the casserole and boil, stirring, for 30 seconds to deglaze. Pour in the stock and bring to a boil.
Tip the remaining marinade ingredients out of the sieve onto a piece of muslin and tie up in a parcel to make a bouquet garni. Add to the casserole with the venison. Bring liquid to simmering point, then put the casserole in the oven. Cook for 45 minutes and then add the shallots. Cook for another 1 hour.
Discard the bouquet garni, remove the venison and shallots from the cooking liquid and keep warm. Add the red currant jelly to the liquid and boil on the stovetop for 4-5 minutes to reduce by half. Strain the sauce and pour over the venison. Serve garnished with sprigs of rosemary.