01 - Preheat the oven to 400°F. Butter a medium-sized baking dish.
02 - Bring a large pot of salted water to a boil. Cook macaroni until just al dente, about 1-2 minutes less than package directions. Drain and set aside.
03 - Melt 1 tbsp butter in a large skillet over medium heat. Add leeks and cook, stirring occasionally, for 5-6 minutes until softened but not browned. Remove from heat.
04 - In a medium saucepan, melt remaining butter over medium heat. Whisk in flour and cook for 1 minute to form a roux.
05 - Gradually whisk in milk, stirring constantly to avoid lumps. Simmer gently until slightly thickened, about 5 minutes. Remove from heat and stir in cheddar, parmesan, cream cheese if using, Dijon mustard, salt, pepper, and nutmeg until smooth.
06 - Fold cooked macaroni and sautéed leeks into the cheese sauce. Transfer mixture evenly into the prepared baking dish.
07 - In a small bowl, blend breadcrumbs, remaining cheese, and olive oil. Sprinkle evenly over the macaroni mixture.
08 - Bake for 20-25 minutes until golden and bubbling. Allow to stand for 5 minutes before serving.