Soak dried porcini in hot water for 30 minutes until softened. Drain and finely mince porcinis.
Fry onions in butter until soft (5-6 minutes), then add fresh mushrooms and saute until done. Drain and let cool.
In a heavy bowl, stir all ingredients together until smooth. For a smoother spread, puree with an immersion blender. Spread into a container and chill overnight for best flavor. Serve at room temperature.
Drained porcini water has a lot of flavor, and can be saved for other recipes, like vegetable soup or cream sauces.