Mix together the diced cucumber and avocado with the lemon juice.
Flake the tuna and mix well with the mayo and mustard.
Add the tuna to the avocado and cucumber.
Add salt to taste.
Prepare the salad greens (optional: add olive oil and lemon juice to taste).
Place the tuna salad on top of the salad greens.
Sprinkle black pepper on top.