Preheat oven to 200°F and place 2 nonstick baking sheets in the oven.

Grate the onion and potatoes using a box grater or food processor and place in a colander to drain.

In a large mixing bowl, beat the eggs and whisk in the flour.

Press the potatoes and onion to extract as much liquid as possible, then add to the egg/flour mixture. Season with salt and pepper.

Mix well, but do not overwork.

In a heavy-bottomed 12-inch skillet over moderately high heat, heat 1 tablespoon of oil and 1 tablespoon of butter until hot but not smoking.

Drop 4 scant 1/4-cup portions of potato mixture into the pan and flatten with a spatula to form four 3-inch pancakes.

Fry until bottoms are golden-brown, 4 to 5 minutes, then turn over and fry until golden-brown and crisp, an additional 4 to 5 minutes.

Transfer to paper towels to drain and season immediately with salt and pepper.

Keep warm on baking sheets in the oven while making remaining pancakes.

Using paper towels, carefully wipe out the pan.

Add 1 tablespoon oil and 1 tablespoon butter and fry 4 more pancakes. Repeat with remaining batter, wiping out the pan and adding 1 tablespoon oil and 1 tablespoon butter before each batch.

Serve pancakes hot with applesauce and/or sour cream.