Brush the olive oil on the pineapple spears. This will keep them from sticking to the grill or grill pan.
Place pineapple on the grill and cook for 3 to 5 minutes on each side, or until dark grill marks appear. After both sides have been grilled, remove and let rest for 3 minutes.
Meanwhile, blitz the mint, lime juice and rind, and sugar in a food processor or blender - add a little water if the mixture doesn't come together. It should be a chunky sauce though, so don't add too much water.