Piquant kalamata olives, rich, tangy feta cheese, and oregano give this super-creamy macaroni and cheese just enough Greek flavor to take it out of the ordinary. Feta cheese, because it is cured and stored in brine, is sometimes called a "pickled cheese."
Cook macaroni according to package directions; drain. Set aside.
Meanwhile, preheat oven to 350 degrees F. For cheese sauce, in a medium saucepan cook onion in hot butter until tender. Stir in flour and pepper. Add milk all at once. Cook and stir over medium heat until slightly thickened and bubbly. Add cheeses, stirring until melted. Stir in cooked macaroni and olives. Transfer mixture to an ungreased 2-quart casserole.
Bake, uncovered, for 25 to 30 minutes or until bubbly. Top with feta cheese and oregano. Let stand for 10 minutes before serving.