Cook pasta according to package directions. Meanwhile, in a large saucepan, saute onion and garlic in 4 tablespoons butter until tender. Stir in the flour until blended. Gradually add milk and broth. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat; stir in cheeses until melted.
Melt the remaining butter; stir in bread crumbs and Italian seasoning. Drain pasta; toss with cheese sauce. Sprinkle with seasoned bread crumbs.