Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the butter and oil in a large frying pan, add the onions and fry over a medium heat, stirring occasionally, for 15 minutes or until softened and just beginning to turn golden. Stir in the sugar and fry for 10 minutes, stirring frequently as the onions begin to caramelize and turn a deep golden brown.
Stir in the flour, then add the ale, stock, bay leaves and Worcestershire sauce. Add a little salt and pepper and bring to the boil, stirring. Pour into the slow cooker pot, cover with the lid and cook on low for 4–5 hours or until the onions are very soft.
When almost ready to serve, toast the French bread slices on both sides, sprinkle with the cheese and drizzle with the Worcestershire sauce. Grill until the cheese is bubbling. Ladle the soup into shallow bowls and float the croutes on top.