Coat a buttered shallow 3-quart baking dish with bread crumbs. Toss tortellini with butter and Parmesan cheese. Add Swiss and mozzarella cheese and toss lightly. Turn half of the mixture into dish. Make white sauce by melting butter. Blend in flour. Gradually add milk, stirring constantly until sauce boils 1 minute. Add salt, white pepper, and nutmeg. Top dish with half the white sauce. Repeat layers. Top with bread crumbs and/or Parmesan cheese. Bake at 350° for 25 minutes.