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. Serve over your favorite pasta.