In a small bowl, mix all the dressing ingredients together. Set it aside.
Slice the onion thinly and sprinkle with a generous pinch of salt. Massage the salted onions and let it sit for about 10 minutes. Rinse with water then squeeze the washed onions to get as much moisture out.
Cook pasta according to package instructions. Drain and add a tablespoon of sesame oil to the cooked pasta and toss to coat.
Add tuna (drained), onions (rinsed) and dressing mixture. Mix well.