Preheat oven to 200°C.
Trim sinew from the venison loin and lightly score the skin.
Sprinkle pepper, salt, and most of the rosemary onto a board, reserving one teaspoon for the sauce.
Rub the loin with oil and roll it in the herb mixture to coat lightly.
Heat a frying pan and sear the venison on all sides until browned.
Roast in the oven for 10-12 minutes, then rest for ten minutes.
Deglaze the frying pan with red wine over high heat, then reduce heat and season.
Mix softened butter and flour together to create a beurre manié.
Whisk the beurre manié into the wine sauce bit by bit until thickened.
Stir in the reserved rosemary.
Slice venison on the diagonal, drizzle with sauce, and garnish with rosemary.