Place the fish fillets in a stewpot or a large skillet with the onion, garlic, salt and bouquet garni. Cover fish with wine, reserving ½ cup of wine. Bring the mixture to a boil, cover and reduce the heat. Simmer for 5-8 minutes or until fish flakes easily when tested with a fork. Remove the fish with a slotted spatula to a serving dish and keep warm. While the fish is cooking, pierce a cross in the bottom of each small onion and add to a medium sized skillet with the mushrooms and 1 tablespoon of butter. Add ½ cup of reserved wine and sauté over medium heat until mushrooms and onions are tender, about 10-12 minutes
Melt 2 tablespoons of butter in a small dish and stir in flour with a wire whisk; blend completely. Bring remaining liquid in large skillet to a boil and gradually stir in butter mixture with a wire whisk. Cook down to about half the liquid or 1 cup. (If a smooth sauce is desired, strain liquid before reducing.)
To serve place one fillet on each plate with sautéed mushrooms and 3-4 onions to the side. Drizzle sauce on top of fish. Garnish plate with a mushroom star or fluted mushroom.
French bread that has been toasted can have the flavor enhanced by rubbing a cut clove of garlic across the bread.