Preheat oven to 375°. In a large saucepan, melt 4 tablespoons butter over medium heat. Stir in flour, parsley, salt, thyme and pepper until smooth. Gradually add milk; bring to a boil. Cook and stir for 2 minutes. , Combine potatoes, ham and onion; place half in a greased 2-1/2-qt. baking dish. Top with half of the sauce; repeat layers. , Cover and bake until potatoes are almost tender, 65-75 minutes. Dot with remaining 2 tablespoons butter. Bake, uncovered, until potatoes are tender, 15-20 minutes.