Cook the pasta in salted water. Drain and let cool. Drain the tuna and collect the oil. Cut the peppers into strips and dice the tomatoes.
Finely chop the onions, place in the salad bowl and mix with the tuna oil. Mix the ingredients for the dressing.
Chop up the spaghetti, chop the tuna and fold into the dressing with the vegetables. Season well. Let it steep for 60 minutes, season to taste and garnish with chives.