Cook pasta according to package directions; drain.
Melt butter in a Dutch oven over low heat; whisk in flour until smooth. Cook 1 minute, whisking constantly. Gradually whisk in milk; cook over medium heat, whisking constantly, until mixture is thickened and bubbly. Stir in 2 cups Cheddar cheese and next 3 ingredients. Stir in pasta.
Spoon mixture into a lightly greased 13- x 9-inch baking dish. Top with remaining 1 cup Cheddar cheese. Combine breadcrumbs and Greek seasoning; sprinkle over cheese.