1/2 lb Mushrooms
3/16 ts Salt
1/16 ts Pepper
1/3    Onion; in 1/4-in slices
1 1/3 tb Butter
2/3 tb Flour
1/3 c  Vegetable stock
1/3 ts Dijon mustard
1/16 c  Sour cream; room temperature

1. Melt 2 tablespoons of butter in a pan and add the flour. 2. Mix in the
vegetable stock and heat, stirring all the time until
thickened. 3. Blend in the mustard and remove from the heat. 4. In
another pan melt the remaining butter and add the mushrooms
and onions and saute for 5 minutes until the onions start to soften. 5.
Add the mushrooms to the sauce, and simmer for 15 minutes. Remove
from the heat and stir in the sour cream. 6. Stir in the onions, and
serve.

I sometimes add coriander or lemon grass. Lemon grass goes down well, and
if you add coconut milk/cream instead of the sour cream,lime juice and
tomato paste you get a nice Thai mushroom dish. Thai dishes are often like
a soup, so increase the liquid a bit and serve in a large bowl with two
spoons so you can share it, and a large plate of rice.

Its one of my own creations so report back!