I knew I had this someplace:

Mr Don's Pontchartrain Sauce

1 sweet onion (softball size), peeled and chopped fine
1 cup Chardonnay wine
1 tablespoon butter (no margarine or any substitutions)
1 tablespoon flour
1 cup chicken stock
1/4 cup mushrooms, chopped fine
1 small green pepper, seeded and chopped fine
4 garlic cloves, minced
1/2 cup butter, chopped into 1 tablespoon pieces (no margarine or any substitutions)
salt or black pepper or cayenne pepper or tarragon

1. Cook the Chardonnay and the onion together over a low fire until the wine is reduced and the onion is very soft.
2. Melt 1 tablespoon of butter in another heavy saucepan over medium heat.
3. Add the flour and whisk until blended.
4. Add the stock a little at a time, whisking constantly until blended and thickened.
5. Add the mushrooms, green pepper and garlic. Cover and simmer down until the vegetables are soft.
6. Remove from heat and add 1/2 cup of butter pieces until melted.
7. Add to the Chardonnay (which should now be reduced to a glaze) mixture.
8. Blend well over very low heat.
9. Season to taste with salt (may not be needed), black pepper, cayenne, and tarragon.