In a frying pan, melt the butter and add the olive oil, then turn the heat down add the leeks (and bacon if using) and gently fry the leeks until softened with a pinch of salt sprinkled on top. Remove from the heat and set aside.
Preheat the oven to 180℃
In a saucepan, melt the butter over a medium heat. With a balloon whisk add the flour and whisk continuously for a minute.
Gradually whisk in the milk, a little at a time. Cook for around 5 mins until thickened and smooth.
Remove the sauce from the hob, add the cheese and stir until it is well combined and melted.
Add the leeks (and bacon) and a pinch of pepper to the cheese sauce to combine together. Pour into a small shallow dish, sprinkle with a little cheese and place in the oven for 20-25mins until golden.