This is a low-fat, full-flavour centre piece perfect for any dinner party.
Preheat oven to 200C/400F/Gas 6.
For the venison, trim any sinewy bits from the venison loin and lightly score the skin with a sharp knife.
Sprinkle the black pepper, rosemary, and salt onto a clean chopping board, reserving a teaspoon of the chopped rosemary for the sauce.
Rub the loin with the vegetable oil and roll the loin (including the ends) in the rosemary and pepper mixture to lightly coat. The top should be only lightly covered in the pepper and rosemary mixture.
Heat a large, heavy-based frying pan until hot. Add the venison, fat-side down, and sear for a couple of minutes. Turn over to sear the other side, then sear the ends.
Place the seared venison into a roasting tin and place in the preheated oven for about 10-12 minutes for rare meat, a bit longer for medium-rare or medium. Remove from the oven and allow to rest for ten minutes. This will ensure moist meat.
Meanwhile, make the sauce. Wipe the frying pan that the venison was seared in clean of large bits of rosemary or pepper, but don't remove the cooked, browned bits. Heat the pan on the hob and, when hot, add the red wine to deglaze. The wine will lift off the browned bits, which will help to flavour the sauce. Cook over a high heat for a minute or two, then reduce the heat. Add a pinch of salt and a bit of freshly ground black pepper.
Make a beurre manie by mixing together the softened butter and the flour in a bowl. This will be used to thicken the sauce.
Add a teaspoon or two of the beurre manie to the red wine and stir into the sauce until it thickens. Add another small piece or two, stirring as you go, until the sauce has the desired drizzling consistency. (You can keep any beurre manie you don't use covered in the fridge for a week or so.)
Stir in the reserved finely chopped rosemary at the end.
To serve, slice the venison loin on the diagonal. Drizzle with the red wine sauce and top with a sprig of rosemary.
