Combine the cream cheese, lemon juice, red onion, and basil in a bowl until completely blended.
On a piece of waxed paper, arrange the smoked salmon slices in a rectangle about 3 inches wide and 12 inches long, slightly overlapping each piece. (If the salmon slices are cut irregularly, simply piece them together like a puzzle.)
Spread a thin layer of the cream cheese mixture on the salmon, sprinkle with the capers, and roll up like a jellyroll.
Refrigerate for 1 hour to firm, then cut into 1/2-inch slices. Place each pinwheel atop a cucumber round to serve.
Recipe courtesy of George Stella's Livin' Low Carb: Family Recipes Stella-Style, Simon and Schuster, 2005