Cut the ends of the green beans, and halve them.
Cut the onion into half rings.
Bring water to the boil with some salt and olive oil.
Boil the pasta as indicated on the package.
Add the green beans 6 minutes before the pasta should be ready. They have to cook for 6 minutes, and then they are both ready at the same time.
Meanwhile roast the pine nuts in a dry pan.
Drain the pasta and the green beans, and immediately cool them with cold water.
Mix all ingredients (pasta, green beans, onion, tuna, pine nuts, arugula) in a bowl.
Finish with balsamic vinegar and olive oil and season with salt and pepper.