How to Make It
Rinse mushrooms and pat dry with paper towels. Trim and discard any discolored areas and tough stem ends (whole stems for shiitakes). Working in batches, pulse mushrooms in a food processor until finely chopped (or chop with a sharp knife).
In a 10- to 12-inch nonstick frying pan over medium heat, melt butter. Add onion and stir often until limp, about 5 minutes. Add chopped mushrooms and sprinkle evenly with salt. Stir often until juices have evaporated and mushrooms are beginning to brown, about 10 minutes; remove from heat. Grind in pepper to taste.
In a glass measure, stir together sour cream, vinegar, and tarragon. Pour into mushroom mixture and stir to mix. Serve warm or at room temperature.