Heat oil in a medium saucepan over moderate heat. Cook shallots and mushrooms until golden. Add garlic and tomato paste and saute. Add wine, turn heat to high, and reduce by half. Add brown stock and reduce again by a quarter. Whisk in pimento puree. Break butter into small pieces and whisk into sauce until smooth. Adjust seasonings and strain through a fine sieve. Serve warm with lamb.
c.1997, M.S. Milliken & S. Feniger, all rights reserved