In a small bowl, sprinkle the gelatin over the lemon juice and 1 tablespoon water; let stand 5 minutes. Whisk mixture into the boiling water. Transfer to a food processor. Add the onion and horseradish; puree until smooth. Add the salmon, creme fraiche, hot-pepper sauce, and paprika; season with salt and pepper. Process until smooth.Advertisement
In a medium bowl, whip cream to stiff peaks. Fold salmon mixture into cream. Using a pastry bag fitted with an Ateco #10 plain pastry tip, pipe the mousse into the tartlet shells. Chill tartlets until they are set, about 30 minutes. Garnish with chopped salmon and sprigs of dill.