Slice onion and separate into rings; place in a greased 1-qt. baking dish and set aside. In a small saucepan over low heat, melt the butter. Stir in the flour and salt until smooth. Gradually add milk; bring to a boil over medium heat. Cook and stir for 2 minutes. Remove from the heat; stir in cheese until melted. Pour over onions. Bake, uncovered, at 350° for 45-50 minutes or until onions are tender and cheese is browned.