For the snapper, heat a griddle pan until smoking. Rub the snapper fillet with oil and season, to taste, with salt and freshly ground black pepper.
Place the snapper onto the hot griddle and char-grill for two minutes on each side, or until cooked through.
To make the pineapple salsa, place all of the salsa ingredients into a large bowl and mix together well.
To serve, place the salsa in the centre of a plate with the snapper fillet on top and garnish with the salad onions.