The sauce can be made several days ahead. Refrigerate for 4-5 days or freeze if keeping for longer.
2 teaspoons olive oil
small knob of butter
1 small onion, finely chopped
4 medium button mushrooms, thinly sliced
2 cloves garlic, crushed
1 teaspoon finely chopped rosemary
1 teaspoon sugar
2 tablespoons balsamic vinegar
1 cup red wine
2 cups good beef stock
1 tablespoon butter at room temperature
1 tablespoon plain flour
Serve with Rack of Lamb with Red Wine and Balsamic Sauce.
Sauce: Heat the oil and butter in a medium saucepan. Add the onion, mushrooms, garlic and rosemary with a good pinch of salt, cover and cook until the onion is very tender. Add the sugar and cook for another two minutes to caramelize the onions. Don’t let the onions burn. Add the vinegar and stir to deglaze the base of the saucepan. Add the wine and stock, bring to the boil then reduce the heat and cook at a steady simmer for 30 minutes or until reduced by half. Strain into a clean saucepan, pushing gently to release all the liquids. Discard the solids. Season.
To finish: Put the butter and flour in a small bowl and mash together with the back of a spoon to make a smooth paste. Gradually whisk small pieces into the simmering sauce until it is smooth and glossy. Simmer for 5 minutes to cook out the flour. Makes about 1 1⁄4 cups.