In a large bowl, combine the prawns, salt, pepper, and chilli flakes. Blend well. Cover and place in the refrigerator.
In a large pot over medium heat, melt 1/2 cup (125g) of margarine and add the onions. Saute the onions until they are translucent.
Stir in the celery and capsicum. Increase the heat to high and stir occasionally for 20 minutes. Add the remaining 1/2 cup (125g) of margarine and bring to a boil. Fold in the prawns, reduce heat to medium and cook for 10 minutes.
Mix the cornflour and water together and slowly pour it into the pot; just covering the top of the prawns mixture. Cook on medium/high heat until the celery is soft; about 20 minutes. Add any additional salt, pepper and chilli flakes to taste.
You can reduce the amount of margarine in this recipe by up to 200g if you wish.