FISH: Remove the skin from the fish and pierce the flesh of the fish with a fork. Put fish in a deep dish, season with salt and pepper, add 1/2 cup of mushroom slivers. Marinate for a few hours in champagne.
Heat the oven to 350 degrees. Cook in a well-buttered pan in the oven for 40-50 minutes, adding onions, thyme and parsley, and the rest of the sliced mushrooms and drizzling the mixture with melted butter. After 20 minutes, add the marinade. Once cooked, keep the salmon warm on a heated platter.
SAUCE: Pour the strained cooking liquid in a saucepan, reduce to one-fifth, add cream and reduce until smooth. Meanwhile, prepare a Hollandaise sauce, blending egg yolks, melted butter and the lobster butter, and carefully add to the other sauce. Cover the fish with the sauce and the garnish the serving plate with flower petals. Serve hot.