Start with a pound or more of cremini (baby portobello) mushrooms or mixed 'wild' mushrooms (for the most flavor) or go with standard white mushrooms. Slice three-quarters of them; finely chop the rest.
Heat your largest saute pan on high heat for a few minutes then add 2 T of oil. When the oil is hot but not smoking melt in 1 T of unsalted butter and as soon as the foam subsides add the mushrooms; stir them to mix well with the fat then spread them out in the pan; add a tiny pinch of salt and lower the meat to med-high. Cook the mushrooms without stirring much till they've given up their juices and the juices start to condense, 5-10 min depending on their moisture content. Meanwhile, very finely chop 2 small shallots and mince a small clove of garlic. Mince a teaspoon of fresh thyme leaves or have ready a 1/2 t of dried. (Don't prep the shallot too early as it can sour if exposed to air for too long.) In a measuring cup mix 3/4 c dry un-oaked white wine (I use sauvignon blanc or cheap Taylor 'chablis' for this sort of thing), a couple tablespoons of balsamic vinegar, and if you have some dry sherry available about 2 tablespoons of that as well.
As the moisture in the pan evaporates to nothing the mushrooms will start browning. When it seems that they're about half there stir well and add the shallot, garlic, and thyme and cook till the shallot is just softened, a minute or two.
Add a good dash of soy (about a teaspoon), stir, and let that reduce to nothing. Add the wine mixture and a good pinch of ground white pepper. Reduce the wine mix till it is syrupy, stirring often, several minutes. Here, you can quit as-is, adjusting the salt as desired, or you can make it more saucy by adding either some light beef stock, light chicken stock, or heavy cream (or a combination). Add just a few tablespoons at a time, stir well, and either pull when the liquids reduce to the consistency you desire or add a few more tablespoons to make it saucier. Straight stock is good as is stock with just a little cream. If you go the cream/stock route add the cream first and let it reduce by half (it'll develop a nice nutty flavor), then sauce it up with the stock addition.
I'd go with a full-flavored Swiss but a mild, baby would be fine.
I do not care for flour in sauces like this because I think it muddies the flavors and prevents the intensity I look for. However, if you prefer a more gravy-like finish do this: Make your wine mix, as noted, but reduce it separately in a small pot while you slice the mushrooms--get it to a syrupy consistency which will be just a few tablespoons. Add about a cup of stock and 2 tablespoons of heavy cream to the syrup in the pot; keep lukewarm. When the shallots have softened and you've evaporated the splash of soy, sprinkle about 2 tablespoons of flour over the mushrooms and stir well. Cook about 2-3 minutes then, pouring in a slow, steady stream and whisking at the same time, add the contents of the pot to the mushroom mix. Whisk well and allow to thicken a bit; remove from heat but keep warm for serving.