1. Preheat oven to 325 degrees. Peel potatoes. Using the larger holes of a box grater grate potatoes and onions. Transfer to a large kitchen towel. Twist towel over a sink and squeeze firmly to wring out as much liquid as possible.
2.Whisk breadcrumbs, salt, baking powder, pepper, 2 eggs and chopped spinach in a medium bowl. Add grated potatoes. Using your fingers, mix until well coated. Mixture should be wet and thick. Mix in remaining egg if mixture is too dry and doesn’t hold together.
3. Line a large rimmed baking sheet with several layers of paper towels. Set a wire rack inside another large rimmed backing sheet and set aside.
4. Heat about 4tbsp of oil (oil should measure about 1/8 inches deep) in a 12-inch nonstick skillet over medium heat. Working in batches drop large spoonfuls of mixture into pan, pressing gently with the back of a spoon to flatten slightly.
5. If mixture becomes watery between batches, mix to incorporate, do not drain.
6. Cook latkes, occasionally rotating pan for even browning, until golden brown and cooked through (2 1/2 – 3 minutes per side).
7. Transfer latkes to paper towel-lined baking sheet to drain, then transfer to prepared wire rack. Place sheet with latkes in the oven to keep warm and crisp while cooking remaining latkes.
8. Serve warm latkes with sour cream.