In a large saucepan, cover the potatoes with water. Bring to a boil over high heat. Lower the heat and simmer, covered, for 12 to 15 minutes or until potatoes are almost tender. Drain.
Preheat the oven to 350°F. Meanwhile, in a small saucepan, combine the onion and the 1⁄4 cup water. Bring to a boil. Lower the heat and simmer, covered, for 5 minutes or until tender. Drain. Stir in the salt, pepper, and celery seeds (if using). In a small bowl, whisk together the milk and flour. Stir into the onion mixture. Cook over moderate heat, stirring constantly, until mixture starts to thicken. Cook and stir for 2 minutes or until thickened. Stir in the parsley.
Place half of the potatoes in a lightly greased 1 1⁄2-quart casserole. Cover with half of the milk mixture. Add the remaining potatoes and remaining milk mixture. Bake for 60 minutes or until heated through. Sprinkle with the Cheddar cheese.
Notes
Let them rest When the potatoes are removed from the oven, give them 5 to 10 minutes to cool and reabsorb the cream mixture before serving. This results in creamy—not soupy—scalloped potatoes dish.