Boil potatoes in salted water until tender; drain and cool.
Trim green beans and blanch in boiling water for 3 minutes.
Prepare hard-boiled eggs by boiling them for 10 minutes; cool and peel.
Dice cooled potatoes into bite-sized pieces.
In a large bowl, combine potatoes, green beans, and quartered tomatoes.
Add sliced red onion and black olives to the bowl.
Flake the tuna and spread it evenly over the salad mixture.
Arrange hard-boiled egg quarters on top.
Drizzle with olive oil and season with salt and pepper.
Garnish with fresh basil leaves before serving.