Preparation

1. Put dried porcini mushrooms in a heatproof bowl. Cover with boiling water and let stand for 30 minutes. After 30 minutes, drain mushrooms, strain (through a sieve lined with a coffee filter) and reserve cooking liquid. Chop the mushrooms and reserve.
2. Put 2 tablespoons of the butter in a 3-quart sauté pan over medium heat. When butter has melted, add onions and garlic. Sauté lightly until onions have softened, about 8 minutes. Add fresh white mushrooms in 2 batches. Stir mushrooms over medium-high heat until lightly browned, about 6 to 7 minutes. Reduce heat to medium-low and stir in 2 table- spoons of the flour. Continuously stir mixture for about a minute. Add 1⁄4 cup sherry and scrape the bottom of the pan of all that has stuck to it.
3. If needed, add 1⁄2 cup of the stock to scrape anything that remains. Pour mixture out into a bowl to reserve. Wipe out pan and add the remaining tablespoon of butter. Sauté the cremini mushrooms until lightly browned. Stir in chopped porcini. Add remaining half tablespoon of flour and stir for about a minute. Stir in 2 tablespoons of sherry, dislodging any flavorful brown bits that remain on bottom of pan.
4. Add one-half cup of the stock if necessary. Add the reserved cooked mushrooms, salt, thyme, 1⁄2 cup mushroom soaking liquid, and remaining stock to the sauté pan. Bring mixture to a boil and then reduce to a simmer. Simmer for about 20 to 30 minutes.
5. Insert the hand blender into the mixture making certain the protective guard is submerged. Carefully blend on Low using a circular motion, moving the blender over the surface of the pan and through the gravy. Serve immediately or store sealed in refrigerator for up to 5 days.