Sprinkle gelatin over lemon juice and 1 1/2 teaspoons cold water in a small bowl, and let stand until softened, about 5 minutes. Whisk boiling water into gelatin mixture until incorporated.
Puree gelatin mixture with onion and horseradish in a food processor until smooth. Add salmon, creme fraiche, hot-pepper sauce, and paprika. Season with salt and pepper. Process until smooth.
Whip cream in a medium bowl until stiff peaks form. Fold whipped cream into salmon mixture.
Peel cucumbers, slice 1 inch thick, and scoop out some of the top of each slice to create a small cavity. (Cucumber cups can be covered with a damp paper towel and wrapped tightly in plastic; refrigerate for up to 1 day before filling.) Using a pastry bag fitted with an Ateco star tip #20, pipe mousse into cucumber cups, about 1 tablespoon per cup. Garnish with dill. Chill cucumber cups until mousse is set, about 30 minutes. Serve immediately.
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