Combine first 5 ingredients in a bowl; beat at low speed of an electric mixer until thoroughly blended. Cut each bread slice diagonally in half. Separate salmon into very thin slices (smoked salmon should separate naturally into slivers). Cut salmon into 48 pieces.
Spread 1/2 teaspoon cream cheese mixture onto each piece of bread. Roll a cucumber slice inside each salmon piece; place over cream cheese spread on bread. Dollop 1/2 teaspoon cream cheese mixture over salmon. Sprinkle with onion and pepper; garnish, if desired.