Drain the canned tuna in water well in a colander. Peel the hard-boiled eggs and cut them into pieces. Rinse the half cucumber and cut the flesh into small cubes. Finely mince the red onion and roughly chop the parsley.
Put the drained tuna in a large mixing bowl and flake the pieces of fish with a fork. Add the mayonnaise and Dijon mustard to the bowl. Stir this mixture well until a creamy base forms. Season everything with salt and pepper.
Add the cucumber cubes, minced red onion, hard-boiled egg pieces and capers to the tuna mixture. Gently fold everything together with a spoon so that all ingredients are evenly distributed but the texture of the egg is preserved.
Scoop the tuna salad into a serving bowl or divide the portions into small bowls. Scatter the roughly chopped parsley generously over the top as a garnish. Serve the salad immediately or keep it covered in the refrigerator until ready to use.