Preheat oven to 425°F. Pour oil into a 9- x 13-inch casserole dish.

Finely chop onions using a food processor or very sharp knife. Place them in a large bowl. Shred potatoes in a food processor or with a box grater. 

Add potatoes, sugar, eggs, salt, and pepper to the bowl with the onions. Mix all ingredients together.

Place oiled casserole dish in 425°F oven for 5 minutes. Carefully remove the pan and swirl the oil around to cover the sides. Pour some of it into the potato mixture. Mix well. Spoon potato mixture into the prepared casserole dish and cook for 1 hour, until the top is golden brown. Cool slightly before serving. Garnish with scallions.