Peel and quarter the pineapple, cut out the hard inner stalk. Dice 3/4 of the pineapple very finely, place in a sieve and drain, collecting the juice. Dice the remaining pineapple, puree and squeeze it out in a cloth, also catch the juice.
Peel the onion and dice it very finely. Boil down to 100 ml of liquid in an open saucepan with the pineapple juice and vinegar. In the meantime, halve the peppers lengthways, clean and cut into tiny cubes.
Mix the pineapple cubes with the reduced juice and the peppers and cover and refrigerate until ready to serve.