Melt a stick of butter in a large and heavy skillet, along with a couple tablespoons of olive (or other light) oil to keep it from burning. As it melts stir in a cup of flour. Keep stirring over high heat until the mixture is well browned (about the color of a chocolate bar). The butter should absorb all the flour and be pasty and semi-liquid—add a bit more oil if there isn't enough and it's too dry. If this isn't intended to be vegetarian, add a can of anchovies in oil, oil and all as it adds a bit more oil and a delightful flavor too.
Once the mixture is well browned start chopping up mushrooms (robust kinds that are fairly dark, not very fragile, and will stand up to red wine; morels, black trumpets, wine cap Stropharia, meadow mushrooms, maitake, or honey mushrooms are all great) rather finely and throwing them in. Keep stirring and scraping the bottom of the skillet as you go and keep the heat pretty high as you want to drive off the liquid from the mushrooms, without getting the heat so out of control that you scorch things. You want to add at least several pounds of chopped mushrooms in all, gradually, so they cook down or there won't be enough room in the skillet. If you add them at the proper rate they will shrink down and you will eventually end up with a thick dark brown mass of cooked mushrooms-in-flour-and-butter that's about the consistency (and color) of fudge with lots of nuts in it just before it sets.
At that point remove it from the heat and let it cool down a bit, then add (with lots of stirring) a whole bottle of decent red wine and an equal amount of stock (suit yourself on the type, any kind seems to work. Vegetable broth is fine too, especially if it was made with a couple dried shiitakes and lots of root veggies—or just use water, but it won't be as good).
Put the mixture back on high heat and bring to a boil. It should thicken a bit. Enough to coat a spoon is all you want, which might not be what you expected from that much flour—flour that's cooked that long doesn't thicken up a huge amount—and you don't WANT it to or it would be pasty. Again just thick enough to coat a spoon—in fact if it is any thicker add a bit of water or more stock. Season with a couple tablespoons of soy sauce, additional salt to taste, lots of pepper—and a couple tablespoons of Worcestershire. Depending on the wine used and the mushrooms it may be a bit tannic so will probably need just a touch of sweetening (it should taste rich but not excessively sweet—tread cautiously here!) A tablespoon or two of brown sugar is about right. And then taste it one last time as it might need yet a bit more salt as the flavors marry up, but be careful not to overdo it.
Then turn the heat down and simmer for half an hour, stirring frequently to keep from sticking. At that point it's done and can be spooned over anything that needs gravy, especially something marinated in herbs and olive oil (and the same red wine you used to make the gravy) and then grilled. Or brown mushroom gravy with whole kernel corn and mashed potatoes is a classic. Or braise something in it.
Freeze the rest—it freezes great, although seems to thicken in the freezer so will probably need to be heated with an equal quantity of red wine to reconstitute it when thawed.