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Sauté 250g sliced shallots in a medium saucepan with 4 tbsp olive oil over a high heat for about 3 mins until lightly browned, stirring often.
Season with ground black pepper and add 1 lightly crushed garlic clove and a sprig of rosemary.
Continue cooking for a further 3 mins, stirring often to prevent the shallots burning.
Pour in 5 tbsp balsamic vinegar and cook until evaporated away to a syrup, then pour in 400ml red wine and cook until reduced by two thirds.
Pour in 400ml beef or brown chicken 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.