Melt the butter in a large, heavy-based saucepan over a moderate heat. Add the onions and saute for 8-10 minutes, stirring occasionally, until soft and starting to colour. Add the bacon at this point and stir well, continuing to cook for 2-3 minutes.
Add the stock and milk, bring to the boil, then reduce to a simmer. Cook gently for 10 minutes until slightly thickened. Adjust the seasoning, then ladle into serving bowls.
Garnish with chunks of bread, a sprig of sage leaves and some more black pepper before serving.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.