Put potatoes and onion in a 3 quart sauce pan and add water to just cover the vegetables. Boil potatoes and onion until tender. Meanwhile, melt the butter in a small saucepan and add flour, stirring until smooth and thickened. Add salt, pepper, and canned milk to flour mixture and stir until mixed well. Remove the sauce from heat. When the potatoes and onion are tender (do not drain), stir in the sauce from the small sauce pan. Add the parsley and more salt and pepper, as desired, to taste. Let stand 10-15 minutes before serving.