1. Trim ends off cucumbers. Split each cucumber in half lengthwise. Scoop out center of cucumbers, leaving a 1/4- to 1/2-inch-thick shell. Cut into 24 (1 1/2-inch-long) pieces. Reserve any remaining cucumber for another use.
2. Combine yogurt, salmon, and onion in a food processor; process until almost smooth, scraping sides of bowl as needed. Spoon 2 teaspoons salmon mixture into each cucumber piece.