How to Make It
In a food processor, combine the scallops, egg whites, 1/2 teaspoon of kosher salt and 1/4 teaspoon of white pepper and process until smooth. With the machine on, add the cream in a thin stream. Scrape the puree into a bowl and refrigerate until chilled, about 30 minutes.
Tear four 12-inch sheets of plastic wrap. Spoon one fourth of the puree onto each sheet in a 6-inch square. Slide the green olives lengthwise onto four 6-inch bamboo skewers. Lay one skewer down in the center of each square. Roll the puree around the olives skewers and tightly roll up the plastic wrap to form cylinders; twist the ends and secure with twist ties. Wrap each cylinder in another sheet of plastic wrap and refrigerate until chilled, about 30 minutes.
Bring a large, deep skillet of water to a simmer. Add the cylinders, cover with an inverted heatproof plate to keep them submerged and poach over moderately low heat until firm, 15 to 20 minutes. Let cool, then refrigerate until completely chilled.
Meanwhile, in a medium bowl, mash the garlic to a paste with a pinch of salt. Whisk in the mayonnaise, paprika and lemon juice, then whisk in the olive oil in a thin stream.
Preheat the oven to 350°. Brush the bread on both sides with the melted butter. Using a 2-inch round cutter, stamp out as many rounds as possible from the bread. Transfer the rounds to a baking sheet and toast until golden, about 15 minutes.
Unwrap the cylinders and remove the skewers. Using a sharp knife, cut the cylinders into slices about inch thick. Place a piece of black olive in the center of each green olive. If desired, use a slightly smaller round cutter to stamp the mousse slices into perfect rounds. Spoon the sauce onto the toasts, top with the mousse rounds and serve.