1. Wash and pare potatoes. Grate potatoes on coarse grater into large bowl rilled with ice water. Let stand 15 minutes.
2. Preheat oven to 350°F. Grease inside of 1 1/2-quart baking dish.
3. Drain potato; pat dry with a clean dish towel. Measure about 5 1/2 cups.
4. In large bowl, with electric mixer, beat eggs until thick and light. Stir in the potato, potato flour, salt, baking powder, pepper, onion and 1/4 cup melted butter; mix well.
5. Turn into prepared baking dish. Bake, uncovered, 1/2 hour; brush top with some of melted butter. Bake 45 minutes longer, brushing every 10 minutes with butter, until top is crusty and golden-brown.