In a large skillet, saute mushrooms in some butter; remove them from the pan and place fish fillets in the pan with a bit more butter.
Add wine, salt, pepper and capers.
Bring to boil, reduce heat, cover and simmer gently for 10 minutes (while fish is simmering, begin sauce preparation).
Remove fish to serving platter, top with sautéed mushrooms.
Keep fish warm (I put mine on a cookie sheet, cover it with foil, and keep warm in a 250 degree oven).
Cook remaining liquid in pan until it is reduced to ½ cup; strain and reserve ¼ cup for sauce (I like capers, so I do not strain my liquid - I just use it all and increase the flour in the sauce by about a 1/2 tablespoon).
For sauce, melt butter in a saucepan and stir in flour.
Simmer very gently for 3 minutes.
Stir together yolks and cream; add reserved liquid from skillet and stir into flour mixture.