To make ahead: Complete the recipe through toasting the brioche on the stovetop, then transfer the fish, brioche-side up, to a parchment-lined baking sheet. Keep at room temperature for up to an hour, or refrigerate for up to 4 hours, uncovered. When you’re ready to cook, place the fish in a 375 degrees F oven and cook for 3 to 4 minutes on each side.