Cut the onion in half, then peel it and cut it into half-moon shapes around 1cm (just under ½") thick.
Heat half of the oil in a 28cm or 11" frying pan or skillet over a medium heat and then cook the onions for 12-15 minutes until they start to become golden. Stir regularly to prevent them from burning.
Peel the garlic cloves and slice them as thinly as you can; add them to the onions and cook for 1 more minute. Then transfer everything to a 4-litre (4-quart) slow cooker.
Return the pan to a medium-high heat, add the remaining oil and then brown the sausages evenly on all sides. Transfer them to the slow cooker, keeping the oil in the pan.
Return the pan to a medium heat, add the flour and cook in the oil for 2-3 minutes, or until the flour becomes golden.
Remove the pan from the heat and stir in the beef stock until everything is combined, then add the BBQ sauce, English mustard and Worcestershire sauce. Have a taste here and see if you need a pinch of salt.
Pour the sauce over the sausages, add a lid and cook on low for 2-2½ hours, then serve.