1. Oil a 3-cup decorative mold. Place lemon juice in a small saucepan, stir in the gelatin and let mixture stand 5 minutes to soften the gelatin. Stir over medium heat until gelatin dissolves, 1-2 minutes. Let cool a few minutes then whisk in cream cheese and sour cream until smooth.

2. Remove skin and bones from smoked fish. Break up fish pieces with your fingers or a fork to check carefully for any remaining bones. Place fish in food processor with chives, parsley, capers, paprika, and pepper. Process briefly until mixture is combined. Add cream cheese mixture and process to combine.

3. Whip the heavy cream with electric beaters in a clean bowl until stiff peaks form. Stir 1/4 of the whipped cream into the whitefish mixture to lighten it, then gently fold in remaining whipped cream. Spread mixture in prepared mold. Cover and refrigerate until firm, about 3 hours. 

4.  To serve, holding the mold with both hands, immerse just the bottom of the mold in a pan of hot water for just a moment—this will loosen the mousse from the mold. Now you can unmold it onto a plate. Surround mousse with arugula leaves.