Step 1

Preheat oven to 325°. Peel potatoes. Using the large holes of a box grater or the grater disk on a food processor, grate potatoes and onions. 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 to loosen. Gather towel; wring out once more.

Step 2

Whisk breadcrumbs, salt, baking powder, pepper, and 2 eggs in a medium bowl to blend. Add potato mixture. Using your fingers, mix until well coated. Latke mixture should be wet and thick, not soupy; mix in remaining egg if mixture is too dry and doesn't hold together.

Step 3

Line a large rimmed baking sheet with several layers of paper towels. Set a wire rack inside another large rimmed baking sheet; set aside. Heat 2 tablespoons schmaltz, if using, and 2 tablespoons oil (or 4 tablespoons oil if not using schmaltz; fat should measure about 1/8 inches) in a 12 inches nonstick skillet over medium-high heat. Drop a small amount of latke mixture into pan. If the fat sizzles around the edges, it's ready. (Do not let fat smoke.)

Step 4

Working in batches and adding more schmaltz and oil to skillet as needed to maintain 1/8 inches fat, drop large 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; do not drain.)

Step 5

Cook latkes, occasionally rotating pan for even browning, until golden brown and cooked through, 2 1/2-3 minutes per side. (If small pieces of potato floating in the oil start to burn, carefully strain out.)

Step 6

Transfer latkes to paper towel-lined baking sheet to drain, then transfer to prepared wire rack. Place sheet with latkes in oven to keep warm and crisp while cooking remaining latkes.

Step 7

Serve warm latkes with applesauce and sour cream.