1 c  Balsamic vinegar
4    Cl Garlic; sliced
4 pn (2-inch) fresh rosemary
2 tb Butter; optional

Put all the ingredients except the butter into a saucepan and bring to a
boil. Reduce the heat and simmer until it is reduced by half, about 10
minutes. The sauce is ready when it coats the back of a spoon. Add the
optional butter and stir well.