Serving small cups of soup as hors d'oeuvre is a nice way to warm up a winter gathering.
Have your fishmonger remove gills and any traces of blood from heads and wash bones, cut to fit in a 12-quart stockpot, thoroughly.
Place onion, fennel, carrots, and celery in a food processor; pulse until medium fine. Over medium heat, melt butter in stockpot. Add processed vegetables, and cook, stirring occasionally, until tender, 8 to 10 minutes.
Cut 6 stalks of lemongrass in half lengthwise; set aside. Remove and discard tough outer layers of remaining 4 stalks; cut into very thin slices crosswise, and set aside.
Add fish heads and bones to stockpot; raise heat to medium high. Cook, stirring occasionally, for 3 to 5 minutes. Add sake, ginger, reserved lemongrass stalks, parsley, cilantro, peppercorns, and 2 1/2 quarts water. Reduce heat to low, skim any scum that has risen to surface, and simmer 25 minutes. Remove from heat; let sit for 10 minutes. Pass through a strainer lined with a double layer of dampened cheesecloth; discard solids. Skim off fat.
Add the crabmeat, reserved lemongrass slices, and mushrooms; season with salt. Return soup to medium heat, and simmer 10 minutes. Pour soup into 12 very small vessels, such as sake cups. Garnish each with a cilantro leaf, and serve. Refill as necessary.
To save time, you can purchase good-quality fish stock at better fish markets.