Pre-heat the grill to hot.
Whisk together the grated lime zest, lime juice and caster sugar until the sugar has dissolved. Coat the pineapple rings in the lime juice mixture, then arrange spaced apart on lined grilling trays. Grill until lightly caramelized on top.
Arrange stacked on serving plates and pour the lime juice marinade on top. Garnish with the pared lime zest and some ground black pepper before serving.