Melt the butter in a large pan, add the bacon and fry it until it is just starting to colour. Add the leeks and potatoes, then stir well until they are glistening.
Add the stock and bay leaves, season and bring to the boil. Partly cover and simmer for 15 mins, until everything is cooked. Fish out the bay leaves, then purée the soup in batches in a food processor or blender. Return to the pan and stir in the milk. Reheat gently and season to taste. Add more stock or water if the soup seems too thick (this will depend on the size of your potatoes). Sprinkle with parsley to serve.
Make the toasts: heat oven to 200C/fan 180C/gas 6. Cut the baguette into thin diagonal slices. Mix the oil and mustard together, then brush over both sides of the bread. Spread them on a large baking sheet and bake for 10 mins. Serve with the soup for dipping. They can be baked earlier in the day and served cold or warmed through in the oven.