Heat the oil in a large casserole or wok set over a medium-high heat. Add the chopped onions and cook slowly, turning down the heat to low-medium, and stirring every four to five minutes until softened and starting to colour lightly and become oil – this will take at least 15 minutes.
While the onions are cooking, crush the garlic cloves and ginger in a pestle and mortar, or blitz in a food processor.
Once the onions are ready, add the crushed garlic, ginger and coriander stalks and fry for a few minutes until fragrant. Add the spices and creamed coconut, breaking it up into smaller pieces as you stir. It should melt in.
Finally, add the fish sauce and the prawns and cook over a medium heat for about five minutes, or until the prawns are cooked through.
Serve hot on steamed rice and garnish with coriander leaves.