Strip the leave off the rosemary and chop them finely with the onion, garlic and prosciutto. Heat half the butter in a large heavy saucepan with a lid.
Add chopped mixture and soften over moderately low heat for 5 minutes. Season with pepper. Increase the heat, add the venison and cook for 10 minutes, or until brown on all sides.
Put the stock in another saucepan and bring to a boil, then reduce the heat and keep at a low simmer.
Increase the heat under the venison, add the vinegar and cook until the liquid becomes thick and syrupy. Pour in red wine. When that becomes syrupy, stir in half of the simmering stock.
Add the cloves, juniper berries, allspice and bay leaf and simmer for 1 hour, covered, stirring occasionally and adding hot water, if necessary, to maintain the liquid level.
Meanwhile, melt remaining butter in a saucepan. Stir in the flour and cook over moderately low heat for 1 minute. Slowly stir in the remaining stock and cook until the sauce thickens slightly.
Stir sauce into the venison casserole, then add the brandy (or Marsala). Uncover pan and simmer for a further 20 minutes. Taste for salt and pepper.
Combine the lemon zest and parsley and sprinkle over the top before serving.