1. Preheat oven to 350ºF.
2. Wash and dry the potatoes, do not peel. Using the large side of a box grater, grate the russet and sweet potato. Grate the onion. Transfer to a colander and press firmly with the palm of your hand to remove excess moisture. Toss and press again.
3. Transfer potato mixture into a large mixing bowl. In a small bowl, mix together the baking powder, salt and flour. Add the egg and the flour to the large mixing bowl; toss to coat.
4. Line a baking sheet with parchment paper and spray lightly with cooking spray or oil. Drop 3 tablespoons of latke mixture onto the baking sheet and shape each one into a 3-inch wide and ¼-inch thick pancake. Continue with remaining mixture. Leave 1-inch between each latke.
5. Transfer baking sheet to the oven and bake for 15 minutes. Flip, increase oven temperature to 425ºF and continue to bake until latkes are a deep golden colour, about 10 minutes more.
6. Serve latkes fresh from the oven or room temperature with applesauce or sour cream.