Heat the olive oil in a large, heavy-based saucepan set over a medium heat. Sweat the onion and garlic for 4-5 minutes, stirring occasionally. Add the spices, sugar and a little salt and continue to cook for a few minutes. Add the chopped tomatoes and bring to a simmer. Remove from the heat and set to one side.
Pulse the prawns in a food processor until finely chopped. Remove to a bowl and add the breadcrumbs, egg and seasoning, mixing well with your hands to combine. Shape into small balls using a 2 tablespoons of the mixture for each ball. Carefully arrange in the sauce and bring the sauce back to a simmer over a medium heat. Simmer for 4-5 minutes until the balls are cooked.
Remove and adjust the seasoning if necessary. Spoon into serving bowls and garnish with a sprig of parsley leaves.