Combine first 5 ingredients in a bowl; beat at low speed with an electric mixer until smooth.
Cut each bread slice in half diagonally. Separate salmon slices and cut into 34 strips. Spread 1/2 teaspoon cream cheese mixture onto each bread triangle. Using a vegetable peeler, cut cucumbers into 34 thin slices. Roll a cucumber slice inside each piece of salmon; place over cream cheese spread on bread triangles. Dollop 1/2 teaspoon cream cheese mixture over salmon. Sprinkle with onion and pepper; garnish, if desired.