Place the onions, butter, Italian seasoning, garlic, bay leaf, sugar and wine in the slow cooker, stir and season to taste. Cook on high for 10 hours. Add the stock and Worcestershire sauce and stir well.
Arrange the slices of bread on top of the soup and sprinkle with the grated cheese. Cook under a preheated grill for 8–10 minutes until golden brown. If your slow cooker pot is not ovenproof, transfer the soup to an ovenproof dish before topping with the bread and cheese.
