For the marinade: Combine 3/4 cup lemon juice, 1/4 cup soy sauce, 1/4 cup water, 1 tablespoon ginger, and 1/2 teaspoon garlic, and stir well.
Cut four 15-inch squares of parchment paper or aluminum foil; fold squares in half, forming a triangle; crease firmly. Place on a large baking sheet; set aside.
Place fish fillets in a 9 inch square baking dish. Pour Soy-Ginger Marinade over the fillets; cover tightly and marinate in the refrigerator for 10 minutes.
Melt the unsalted butter over low heat. Simmer until the fat rises to the top and the milk solids sink to the bottom. Skim off the white froth that appears on top. Strain off the clear, yellow butter, keeping back the sediment of milk solids. (Clarified butter)
Place the clarified butter in a large skillet. Add the mushrooms, garlic, and ginger, and saute until the mushrooms are tender. Add salt and jalapeno pepper, and saute 1 additional minute.
Remove the fillets from marinade; discard the marinade. Place 1 fillet on half of each parchment triangle near the crease. Arrange mushroom mixutre and sliced green onions evenly over fillets. Dot with 1/4 cup butter. Fold over the remaining halves of parchment triangles. Starting at one end of each triangle, pleat and crimp the edges together to seal securely.
Bake at 375 degrees for 7 to 8 minutes or until the bags are puffed and lightly browned and fish flakes easily when tested with a fork. Serve with rice.