Place bacon in a large skillet over medium-high heat. Fry for a few minutes, then add the leeks. Cook and stir until bacon is crisp and leeks are tender. Set aside.
Melt the butter in a saucepan over medium heat.
Whisk together the flour and milk, then pour into the saucepan with the butter. Bring to a boil and cook, stirring constantly, until thickened.
Remove from the heat and stir in the feta cheese.
Mix in bacon and leeks, then season with pepper to taste.
