In a saucepan, melt 4 tablespoons of butter. Stir in flour, parsley, salt, thyme, pepper, and minced onion until smooth. Gradually add milk and bring to a boil over medium heat. Whisk to keep smooth. Cook and stir for 2 minutes.
In a greased 2.5 quart casserole dish, layer potatoes and ham with sauce spooned over. Repeat layers (usually 3-4). Top with the remaining sauce.
Cover and bake for 65-75 minutes or until potatoes are almost tender. Dot with remaining butter and bake, uncovered, for 15-20 minutes longer or until potatoes are fully tender.