Combine onion and cider vinegar in a small dish. Let stand for at least 5 minutes.
Meanwhile, mix together mayonnaise, lemon juice and pepper in medium bowl. Break up tuna and add to mayo mixture, along with cucumber and kidney beans. Drain onions in colander and rinse well. Add to tuna and stir to combine. Serve as a sandwich on whole-grain toast or as a salad with mixed greens and bread on the side.