2 hr 30
Put the reserved prawn shells and heads and the stock in a small saucepan and simmer, covered, over a medium-low heat until required.
Meanwhile, heat the oil in a frying pan over a high heat. Reduce the heat to low, add the sausage and fry, stirring, for 10 minutes until it gives off its fat. Use a slotted spoon to transfer the sausage to the slow cooker.
Sprinkle the flour into