Citrus-Baked Halibut Fillet with Oyster and Champagne Velouté

My advice for cooking this dish is to prepare the sauce first of all and then prepare the fish ready to go into the oven. As fresh white fish will spoil if it is not served immediately, time the cooking of the fish to coincide with the serving time. The sauce can be prepared in advance and reheated before serving. The oysters should be added at the very last minute.