Preheat the oven to 350°. In a medium skillet, melt 2 tablespoons of the butter. Add the onion and cook over moderately high heat, stirring, until translucent, about 5 minutes. Add the garlic, chopped porcini and 2 tablespoons of the reserved porcini soaking liquid and cook over moderately low heat for about 5 minutes. Stir in the rosemary, thyme and sage and scrape the mixture into a large bowl; let cool.