In a saucepan melt butter with oil over moderate heat until foam subsides and sauté shallots and mushrooms over moderately high heat, stirring, until liquid mushrooms give off is evaporated and mushrooms begin to brown. Add Sherry and boil until evaporated. Stir in broth and simmer 15 minutes, or until mixture is reduced to about 2 1/2 cups. Stir in chives or scallion greens and season soup with salt and pepper.