Preheat oven to 200°. Peel potatoes. Grate potatoes and onions on the grater disk on a food processor or box grater. Transfer to a large kitchen towel.
Gather ends of towel; twist over sink and squeeze firmly to wring out as much liquid as possible. Open towel, toss mixture around, gather towel and wring out once more. This is the most difficult but most important step! The latkes won't fry up well if they're overly wet.
Whisk eggs, breadcrumbs, salt, baking powder, and pepper in a medium bowl. Add potato mixture. Mix until well coated. It should be wet and thick but not soupy.
Set a wire rack inside another large rimmed baking sheet and place in the oven.
Heat about 2 tablespoons oil in a 12 inches nonstick skillet over medium-high heat. Drop a small amount of latke mixture into pan. Smoosh the center down a little so they fry evenly. You don't want them to be too chubby.
Cook latkes, occasionally rotating pan for even browning, until golden brown and cooked through, 2-3 minutes per side. Transfer to wire rack in oven to keep warm and crisp while cooking remaining latkes.
Working in batches, add more oil to skillet as needed, drop spoonfuls of mixture into pan, pressing gently with the back of a spoon or spatula to flatten slightly. (If mixture becomes watery between batches, mix to incorporate)
Serve latkes while warm with applesauce and/or sour cream.