Cook noodles and drain. Set aside. Saute onions in butter until tender. Blend in flour, salt, pepper, and mustard. Cook over low heat until smooth and bubbly. Remove from heat and stir in milk. Heat to boiling. Stirring, boil 1 minute. Stir in Swiss cheese and ham. Add noodles. Turn into a 1 1/2 quart ungreased casserole. Bake uncovered 20 minutes at 375 degrees.