Mince mushrooms and shallots. Saute in 2 tablespoons butter. Allow liquid from mushrooms to cook down.
Melt remaining 2 tablespoons butter in separate pan; stir in flour to make roux (paste). Add chicken stock, cream, cayenne, nutmeg, salt and white pepper. Cook until mixture becomes quite thick. (Don't let heat get too high.)When mixture is thick add mushroom mixture.
Trim crusts from bread. Spread some mushroom mixture on one slice of bread, add second slice, more mushroom mixture, then add third and final slice of bread. Don't spread mushroom mixture to edges, or it will ooze out. There should be enough mushroom mixture to make 7 triple-decker sandwiches.).
Preheat oven or toaster-oven to 400°F Toast sandwiches 10 to 12 minutes, or until they are slightly golden in color. Cut each sandwich into three fingers and serve warm.