Cut the palm hearts into slices (not too thin, otherwise they will fall apart easily). Finely dice the onion. Put the palm hearts, onions and crabs in a bowl, season with salt and pepper. Add oil and a strong dash of vinegar. Mix everything, cover with foil and leave in the refrigerator overnight (at least 2 hours).
Just before serving, fold in the avocado (cut into fine pieces) and season to taste again.