Place the sardines in a roasting pan with 1 1/2 cups of the white wine and the court bouillon. The sardines should be covered completely. If not, add more water. Bring the liquid to a simmer. Remove pan from the heat, cover, and stand until the poaching liquid has cooled, 25-30 minutes. Transfer the sardines to a warmed serving platter. Strain the cooking liquid. Melt 1/4 cup of the butter in a large saucepan and cook the onions and mushrooms until softened, about 5 minutes. Pour the remaining wine and 1 1/2 cups of the strained poaching liquid into the saucepan, and bring to a boil. Boil hard until reduced by one-third. Strain the sauce and return it to a saucepan. Whisk in the butter, a little at a time, until the sauce thickens. Do not boil the sauce. Pour the sauce over the sardines. If sauce is not sufficiently thickened before adding butter, beat in 1 tablespoon flour dissolved in 1 tablespoon of the sauce. Boil for 2 minutes.
