Combine the mushrooms, onion, garlic, and walnuts in a food processor or blender and pulse until the mixture forms a coarse paste. Heat the oil in a medium-size skillet over medium heat, add the mushroom mixture, and season with salt and pepper. Cook, stirring, for 1 minute. Add the wine and cook, stirring, for 1 minute more. Remove from the heat and let cool to room temperature before using.
Yield: about 1 cup