In a medium saucepan over medium heat, heat the oil. Add the shallots, garlic, thyme, and bay leaf. Cover and cook until the shallots are translucent, 4 to 5 minutes.
Add the mushrooms, cover and cook for 1 to 2 minutes or until most of the liquid has evaporated. Add the vinegar and sugar and reduce until almost dry, about 5 minutes more. Add the wine and port and reduce until syrupy, about 15 to 20 minutes. Add the stock and simmer until reduced by half and thickened slightly, about 20 minutes more.
Strain the sauce into a medium saucepan. Discard the solids. Over low heat, stir in the butter. Season to taste with salt and pepper. Do not allow the sauce to boil or it may break.
Makes about 3 cups.
Chef's Tip: Do not whisk the butter into the sauce. Stir it with a spoon. Whisking adds air and clouds the sauce when it should be shiny.
Recipe courtesy of Laurent Tourondel, BLT New York.