Cut the onion into cubes and sauté in butter until translucent, then add the risotto rice and deglaze with the juice of the 2 limes. Gradually deglaze alternately with the vegetable stock and coconut milk. Only add as much as the rice can absorb and continue to simmer while stirring constantly. Finally, add the zest of the 2 limes. Fry the prawns separately in a pan and then either add them to the risotto on a skewer or garnish individually on top.