A young chef I know in London passed this recipe along to me, and I’ve used it frequently ever since- Adding milk at the end of the preparation definitely makes the difference.
Preheat the oven to 400 degrees. Spray a 13 x 9 x 2 inch baking dish with cooking spray.
Melt the butter in a large saucepan and cook the leeks, covered for 10 to 12 minutes, or until they are softened but have no color.
Uncover the pan, sprinkle the flour over the leeks and continue to cook for about 1 minute longer, stirring constantly. Add 3 1/2 cups of milk, bring to a boil, reduce the heat and stir in 3 cups of the cheese and the mustard. Remove the pan from the heat and stir until the cheese melts and the sauce is smooth. Add the salt and season with pepper to taste.
Meanwhile, cook the pasta according to package directions so that it is al dente. Drain well, add the pasta to the cheese sauce, and then gently stir the remaining 1/2 cup milk. Pour the pasta into the prepared baking dish, top with the remaining cup of cheese and bake for 25 to 30 minutes until just golden brown. Serve hot.