Preheat the oven to 180°C.
In a saucepan, bring some salted water to the boil and blanch the leek pieces for 2 minutes to softened them. Remove from the water and place in an ovenproof dish, season with some salt and pepper.
Next, prepare the sauce.
Melt the butter with the bacon pieces for 2 minutes until the bacon is caramelised.
stir in the flour and cook for a minute or so, then add the milk.
Whisk quickly until the mixture becomes a smooth and silky sauce. Remove from the heat and stir in the cheese. Check the seasoning and pour the mixture over the leeks.
Sprinkle some breadcrumb over to help give extra crunch.
Bake in the oven for 15 minutes until the sauce is bubbling and golden.