Heat the oil in a frying pan, add the sausages and fry over a medium heat for 5 mins, turning until browned on all sides, but not cooked through. Remove the sausages from the pan and transfer to the slow cooker.
Add the onions to the pan and fry over a medium heat for 5mins or until softened. Add the sugar and fry, stirring for a few more mins.
Stir in the flour, then gradually add the stock. Add the tomato puree, bay leaf and salt and pepper, bring to the boil, stirring continuously.
Pour over the sausages, cover with the lid and cook on high for 4 hours.
Serve in a giant Yorkshire pudding with vegetables or mash potato.
