[I rarely serve mushrooms on their own, but this preparation is worth making an exception for. — Matthew]
Start with at least a pound of mushrooms, remembering that they will shrink substantially when cooked. Any common type of fresh mushroom can be used; most recently I used a mix of cremini, portobello, and shiitakes.
Cut the mushrooms into roughly equal-sized pieces; I cut the cremini caps in half, and sliced the other caps into long strips.
Melt several tablespoons of butter in a very large skillet over medium heat. Add the mushrooms and cook (stirring occasionally) until they give up much of their moisture, becoming soft and dark.
Optionally add a few finely-sliced scallions while the mushrooms cook.
Peel several cloves of garlic and put them through a microplane grater, or chop very fine. Clear a space in the middle of the skillet, add another tablespoon of butter, and add the garlic, cooking until it becomes fragrant, and then stir to combine it with the mushrooms.
Finally, use a liberal dash of balsamic vinegar to deglaze the pan, stirring until it is absorbed by the mushrooms.
Optionally garnish with parsley.