Step 1In a small saucepan, combine 1 glass Champagne (three-fourths cup), the mushrooms, shallot, garlic, thyme, rosemary, pepper and allspice. Cook until reduced by half, about 4 minutes.
Step 2Using a fine mesh strainer, strain out the herbs, garlic and shallot and discard. Add one-half glass Champagne (6 tablespoons) and all of the sauce allemande and reduce further over low heat, stirring constantly. Add the remaining one-half glass Champagne (6 tablespoons) and continue to stir until the sauce reduces further. This is a thin sauce. To hold the sauce until ready for serving, place it in the top of a double boiler over simmering water. Makes about 1 3/4 cups sauce.
Step 3Heat a barbecue grill or grill pan and brush with olive oil. Lightly brush olive oil on both sides of the fish and sprinkle with the salt and pepper. Place the fish, skin side down, on the grill. Cover the fish and grill until the center is faintly showing pink, about 8 minutes.
Step 4Just before serving, swirl the butter and lemon juice into the sauce. Spoon the sauce over the fish.