Peel and grate potatoes on the finest holes of a box grater into a large bowl. Add flour, sour cream, baking powder, eggs, onions, and salt to taste and stir well to combine. Transfer mixture to a sieve set over a bowl and let drain for 10 minutes.
Preheat oven to 200°. Heat 2 tbsp. of the butter in a large skillet over medium heat. Scoop out about 1⁄2 cup of the potato mixture with your hands and squeeze it gently to remove some of the liquid (it should still be very wet). Form potato mixture into a flat pancake about 5" wide and put into skillet. Repeat with a second portion. Cook until golden brown, 5–6 minutes per side. Transfer pancakes to a rack set over a baking sheet and keep warm in oven. Repeat process with remaining butter and potato mixture. Serve with applesauce on the side, if you like.