Notes: Up to 2 days ahead, cook dressing, drain, and separately cover and chill dressing and cooking liquid. To serve, combine dressing and liquid, heat, then drain.
In a bowl, pour 2 cups boiling water over porcini mushrooms. Let stand until water is cool enough to touch, about 20 minutes. Squeeze mushrooms gently to release grit, then lift from water. Finely chop mushrooms. Carefully pour soaking liquid into another container, leaving grit behind. Add water to soaking liquid to make 1 1/2 cups. Discard gritty residue.
In a 5- to 6-quart pan, combine onions, thyme, nutmeg, and 3/4 cup of the mushroom soaking liquid. Stir often over high heat until onions begin to brown, 10 to 15 minutes. Add remaining soaking liquid, mushrooms, broth, wild rice, and sherry.
Cover and bring to a boil. Reduce heat and simmer until rice is tender to bite and beginning to split, about 1 hour. Drain, reserving liquid for other uses. Season dressing with salt to taste.