Sauté the shallots in a medium saucepan with the oil over a high heat for about 3 mins until lightly browned, stirring often. Season with ground black pepper and add the garlic and rosemary. Continue cooking for a further 3 mins, stirring often to prevent the shallots burning.
Pour in the vinegar and cook until evaporated away to a syrup, then pour in the wine and cook until reduced by two thirds.
Pour in the stock and bring to the boil. Turn down the heat and simmer until reduced by two-thirds again, to around 250ml. Remove the garlic and rosemary. Add a little salt to taste and finally 'monte' (whisk) in a knob of butter. Add any juices from the steaks just before serving.