This recipe appears in Julia Child's TV show cookbook, but not in her two-volume Mastering the Art of French Cooking (if I'm not mistaken). I like its simplicity of preparation, and that it can be prepared in advance to ready-to-bake then baked just before serving. The basic recipe with mushroom filling serves 4 to 6; I recently made a half recipe for two adults and a small child.
Fillings can be varied. I've used this forestière version, but also made it with a salmon and spinach filling (must be careful the spinach isn't still too moist, though), and ham and cheese (I think I recall). The mushrooms reduce with cooking, so I'd suggest about ¾ C to 1 C of alternative fillings, and make sure they won't render liquid and make the ramequin runny and soggy.
Soften the mushrooms and shallots or scallion in the butter and oil. When soft, add the flour to thicken, then the heavy cream off the heat.
Mix and cook, whisking to thicken (make paste). Off heat, beat in
Beat in, one by one:
then