2 tablespoons, or more, butter
1 clove garlic
¼ teaspoon thyme
¼ cup ground dried porcini or other Boletus sp.
1 pint sour cream
½ teaspoon salt or to taste
Slice the onion thinly, chop into smallish shreds. Sauté it in butter until it has turned golden and is starting to brown. Add the garlic, and sauté until the onion is browned, but not burnt. Add the thyme and mushroom, and sauté a few more minutes, until the mushroom has absorbed butter and liquids, and has softened and is just beginning to brown, adding more butter if necessary.
Put the sour cream in a bowl, and mix in the mushroom and onion blend, salting to taste. May be served immediately, but is better if chilled a few hours to overnight, to let the favors meld.