Cut the cucumber crossways into 1.5cm-thick slices. Use a melon baller or small teaspoon to scoop out most of the seeds and a little of the flesh, being careful not to go all the way through.
Combine the salmon, sour cream, onion, wasabi and lime juice in a large bowl.
Spoon the salmon mixture among the cucumber cups.
Top with dill and season with pepper. Serve with lime wedges.