Peel the cucumber if the skin feels tough, slice it into thin rounds, sprinkle lightly with salt and let it rest for five minutes so excess moisture can surface.
Gently squeeze the cucumber to remove the released liquid, ensuring the slices stay crisp and do not water down the salad.
Peel the avocado, cut it into neat half‑moon slices and toss it immediately with the juice of half a lemon to preserve colour and add freshness.
Boil the eggs, cook them for at least two minutes after the water reaches a steady boil, then transfer them to ice water for five minutes to stop the cooking process.
Peel the cooled eggs carefully and chop them into medium pieces that will hold their shape in the salad.
Drain the tuna thoroughly and flake it lightly so it distributes evenly without adding excess liquid.
Combine the cucumber, tuna, chopped eggs, parsley and green onion in a large bowl, mixing gently to keep the textures distinct.
In a small bowl, mix Greek yoghurt, mustard, salt and black pepper until the dressing becomes smooth and cohesive.
Add the avocado with its lemon juice to the salad, pour in the dressing and toss gently to coat all ingredients without crushing the avocado.
Serve the salad immediately to enjoy the crisp cucumber, creamy avocado and tender eggs at their freshest.