Cut onions crosswise into 1"-thick slices and separate into rings. Place rings in a large bowl of cold water and soak for at least 30 minutes to remove any bitterness.
To make dry batter, mix together 2 1⁄4 cups of the flour, cornmeal, cornstarch, baking powder, parsley, and seasoned salt in a large bowl, then cover and set aside.
Pour enough oil into a cast-iron pot to come to a depth of 3". Heat over medium-high heat to 325°–350°. Put remaining 2 cups flour in a bowl. In another bowl, mix together milk and buttermilk.
Drain onions. Working in batches, about 3–5 rings at a time, dredge onions in flour, then dip in milk mixture, then dredge in dry batter. Immediately fry onions in hot oil until crisp and golden, about 2 minutes. Use tongs or a slotted spoon to remove onion rings from oil; drain well on paper towels before serving.