Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan, add the sausages and fry over a high heat for 5 minutes, turning until browned on all sides but not cooked through. Drain and transfer to the slow cooker pot.
Add the onions to the frying pan and fry over a medium heat for 5 minutes or until softened. Add the sugar and fry, stirring, for 5 more minutes or until the onion slices are caramelized around the edges.
Stir in the flour, then gradually mix in the stock. Add the tomato purée, the bay leaf and some salt and pepper and bring to the boil, still stirring. Pour over the sausages. Cover with the lid and cook on low for 6–8 hours or until the sausages are tender.
Serve spooned into reheated ready-made large Yorkshire puddings accompanied with steamed carrots and broccoli or mashed potatoes.