Peruvians are very fussy about the treatment of onions. They love raw onions but not as much as their Mexican cousins, as they find them a little harsh. So to make them less aggressive and the onion flavour less dominating, they purge them briefly in boiling, salted water. Equally important is to cut the onions ‘a la pluma’, or ‘like feathers’. To do this, slice them into very thin half moons, retaining the curve of the onion, so that they look like little curls on the plate. The mellowed onion and piquant lime of the salsa in this dish make a wonderful accompaniment to the seared rich and oily tuna. Source sashimi-grade tuna for the best result.