Note: To save time, you can purchase good-quality fish stock at better fish markets. Or, if using fresh fish, have your fishmonger remove the gills and any traces of blood from heads, cut the bones to fit into a 12-quart stockpot, and wash the bones thoroughly.

Place onion, fennel, carrots, and celery into a food processor; pulse until medium fine. Over medium heat, melt butter in a 12-quart 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 foam that has risen to surface, and simmer 25 minutes. Remove from heat; let sit for 10 minutes. Pour 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.

Serves 10 to 12.