In a large nonstick skillet, melt the margarine. Saute the shallot until soft, about 2 minutes. Add the garlic; cook 1 minute longer. Stir in the mushrooms, sage, salt and pepper; cook, stirring frequently, until the mushrooms are tender, about 6 minutes
Increase the heat to high; cook until almost all the liquid evaporates, about 3 minutes. Add the brandy; cook 1 minute. Stir in the flour; cook, stirring constantly, until smooth. Stir in the evaporated milk; reduce the heat and simmer until thickened, about 7 minutes. Remove from the heat and stir in the sour cream