Place a 9-by-13-inch baking dish on middle oven rack; preheat to 400°F.
Peel potatoes, adding them to a large bowl of cold water as you work.
Shred the potatoes in a food processor fitted with a shredding disk.
Return the shredded potatoes to the water; let stand for 5 to 6 minutes. Drain.
Place the potatoes on a clean kitchen towel; squeeze out as much liquid as possible. Set the potatoes aside.
Shred onions in the food processor. Drain in a fine-mesh strainer, gently pressing to remove excess liquid.
Whisk eggs, salt and pepper in a large bowl until frothy, about 1 minute.
Add potato starch, the shredded potatoes, 3 cups of the shredded onions and 1 tablespoon each parsley and chives. Mix well with your hands until well combined.
Carefully remove the hot baking dish from the oven. Add 4 tablespoons oil to the dish; spread evenly over the bottom and sides of the dish with a pastry brush.
Spoon the potato mixture into the dish; smooth with the back of a spoon. Drizzle evenly with the remaining 2 tablespoons oil.
Bake until the top is browned and the potatoes are tender, 55 to 60 minutes.
Let stand for 10 minutes. Sprinkle with remaining 1 tablespoon each parsley and chives. Serve hot.