How to Make It
In a large skillet, heat the olive oil. Add the mushrooms and cook over moderate heat, stirring, until softened, about 3 minutes. Add the shallots and garlic and cook, stirring, for 2 minutes. Add the stock, season with salt and pepper and simmer until evaporated, about 3 minutes. Let cool to room temperature.
Transfer the mushrooms to a blender and puree. Blend in the butter.
Toast the bread and spread with the mushroom butter. Serve warm.