Blitz the chilli, garlic and 1 tablespoon of the oil into a fine paste in a food processor.
Add the remaining 4 tablespoons of oil to a large frying pan or wok on high heat. Stir-fry the onions and the chilli-garlic paste for 2 minutes until the edges of the onions start to turn translucent.
Add the sugar, salt, tomato puree and water. Stir-fry for a minute to fully incorporate everything together.
Finally, add the prawns to the pan and cook, stirring frequently, for 2 minutes until the prawns are cooked through, then serve.