Both the brioche and the cured ocean trout can be made several days ahead - all you need to do on the day is scramble some eggs. You'll need to begin the cured trout at least a day ahead in any case.
Note You can make the brioche up until they have been shaped and placed in the moulds, then refrigerate them overnight. Bring to room temperature (1-2 hours) then brush with eggwash and bake on the day you want to serve them. Alternatively, they can be baked, cooled and stored in an airtight container in the freezer for several weeks. To serve, thaw at room temperature, then warm in a moderate oven for 10 minutes.