In small saucepan, soften gelatin in cold water; stir over low heat till gelatin is dissolved.
In small bowl, combine tuna, mayonnaise, lemon juice, and finely chopped onion; beat till smooth.
Blend in dissolved gelatin, sour cream, and capers.
Spoon into 3 1/2-cup mold; chill till firm.
Unmold on lettuce-lined platter; garnish with cucumber slices and cherry tomatoes.