In a medium skillet, melt butter or margarine.
Add chopped mushrooms and onions.
Stirring occasionally, cook over medium heat for 3 minutes until vegetables are crisp-tender.
Stir in flour until bubbly.
Stir in cream, salt, pepper, chervil and nutmeg.
Cook and stir over medium heat about 10 minutes until slightly thickened.
Preheat oven to 375°F (190°C).
Spoon mushroom mixture into baked tart shells or toast cups.
Bake for 5 to 8 minutes in preheated oven until warmed through.
Garnish with olive slices.