Pre-heat the grill to hot.
Combine the caster sugar and water in a shallow saucepan. Heat over a medium heat, swirling gently to help the sugar dissolve. Once the sugar has dissolved, cook the caramel until you have a deep golden brown colour, then remove from the heat.
Arrange the walnuts on top of the melon then pour the caramel on top in a zig-zag pattern. Grill for a minute then remove and arrange stacked on serving plates. Sprinkle with the sesame seeds before serving.