Line unpricked pastry shell with a double thickness of heavy-duty foil. Bake at 450° for 8 minutes. Remove foil; bake 5 minutes longer.
Meanwhile, in a large skillet, saute onion in butter until tender. Stir in flour. Gradually add tomatoes. Cook and stir for 2 minutes or until thickened. Add Italian seasoning and salt. Spoon into crust.
In a small bowl, combine mayonnaise and cheeses. Spread over tomato layer. Bake at 350° for 18-25 minutes until cheese is browned and bubbly.