Slice tomatoes in half. Dice cucumber. Finely dice onion. Slice olives into halves.
Add to a mixing bowl tomatoes, cucumber, onion, olives and oregano. Mix together.
Drain and rinse butter beans. Drain tuna.
Add butter beans and tuna to the mixing bowl. Crumble feta into mixing bowl. Add lemon juice, olive oil, and salt and pepper (to taste). Mix together.
Taste test and adjust flavours if needed to your liking - does it need more olive oil? Salt? Pepper? More zingy acidity (e.g. more lemon if on hand? Or sub for a splash of vinegar?). Taste, play, enjoy!