How to Make It
Preheat the oven to 350°. Lightly brush the bread on both sides with the melted butter. Using a 1 1/2-inch biscuit cutter, cut out 4 rounds from each slice of bread. Arrange the bread on a baking sheet and toast for 5 to 6 minutes, or until lightly golden. Let the toasts cool on the baking sheet. Leave the oven on.
Melt the remaining 1 tablespoon of butter in a small skillet. Add the minced scallions and cook over moderate heat, stirring, until softened, about 2 minutes. Scrape the cooked scallions into a food processor and let cool. Add the ginger and pulse until combined. Add the scallops and process to a paste.
Transfer the scallop mousse to a pastry bag fitted with a 1/2-inch round tip or a plastic bag with a corner snipped off. Pipe a rounded 1/2 teaspoon of the mousse onto the toasts. Alternatively, spoon the mousse on the toasts.
Using your finger or a pastry brush, smooth the top of the scallop mousse with the ginger pickling liquid. Bake the scallop toasts for about 5 minutes or until the tops are firm but not rubbery. Let cool, then top half of each toast with the caviar and sprinkle the other half with the sesame seeds. Garnish with the sliced scallion greens and serve the toasts at room temperature.