How to Make It
In a microwave-safe bowl, combine the dried mushrooms and hot water. Cover with plastic wrap and microwave at high power for 2 minutes. Let cool slightly, then drain the mushrooms, reserving the liquid. Finely chop the mushrooms.
Meanwhile, in a soup pot, heat the olive oil until shimmering. Add the leeks and cook over moderate heat until softened, about 5 minutes.
Add the stock, chopped mushrooms, potatoes and thyme to the pot. Slowly add the mushroom soaking liquid, stopping when you reach the grit at the bottom. Cook over moderate heat until the potatoes are almost dissolved, about 20 minutes. Stir in the vinegar and butter and season the soup with salt and pepper. Serve in shallow bowls with crusty bread.