Mix 10 tablespoons mushroom liquid with cornstarch. Add soy sauce to remaining liquid and simmer until reduced by half. Add cornstarch liquid and bring to a boil; whisk in 3 ounces butter. Season with salt and pepper.
Season halibut filets with salt and pepper. Place a 12-inch-long piece of plastic on work surface. Place enough mushroom mixture in center of plastic as needed to cover halibut fillet. Fold plastic over mushroom mixture; pat or roll it with a rolling pin until evenly thick and slightly larger than halibut filet. Unfold and place a halibut filet on top of the mushroom mixture. Gather plastic tightly up and around the halibut as if wrapping a gift basket, until mushroom mixture adheres to top and sides of halibut. Twist ends and cut excess plastic with a scissor. Turn over the halibut on a baking sheet lined with parchment or silpat, 2 inches apart. Repeat with remaining halibut and mushrooms to make 24 portions.
For each serving, to order, cook 1 halibut filet in 300°F oven in its plastic wrapping, mushroom-coated side up, for 20 minutes. Place halibut in center of serving plate. Remove plastic by grabbing the center of the plastic and pulling off. Spoon mushroom sauce around halibut; garnish with chervil.
Recipe by Chef Michel Richard, Citronelle (Washington, D.C.)