Mix lime juice, sugar and chilli. Add salt to taste.
Wash the fish, dry it, cut into cubes with a side of 0.5 cm, pour the resulting mixture and place in the freezer for 15 minutes. Rinse the leek thoroughly, scald, cut into rings and lightly fry in butter for 1 minute, salt.
Put the warm leek on a plate, put the fish on top. Drizzle with remaining marinating mixture.