Stir together cucumber and salt in a medium bowl; let stand for 10 minutes.
Meanwhile, combine yogurt, feta, herbs, lemon zest, lemon juice and pepper in a large bowl.
Drain the cucumbers; spread on a paper towel and pat dry. Add to the yogurt mixture; stir until well coated.
Divide sprouts between 2 bread slices. Spread cucumber salad on top of the sprouts. Top each with an onion slice and another bread slice; cut in half. Serve immediately.
