Using box grater or food processor fitted with grating disc, coarsely grate onion and place in colander set in sink. Coarsely grate potatoes, add to colander, and set aside to drain.

In large mixing bowl, lightly beat eggs, then whisk in flour.

Press potatoes and onion to extract as much liquid as possible, then add to egg/flour mixture. Season with salt and freshly ground black pepper. Using wooden spoon or hands, mix well, but do not overwork.

In heavy-bottomed, 12-inch skillet over moderately high heat, heat 1 tablespoon oil and 1 tablespoon butter until hot but not smoking. Drop 4 scant 1/4-cup portions of potato mixture into pan and flatten with 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; season immediately with salt and pepper. Keep warm on baking sheets in oven while making remaining pancakes.

Using paper towels, carefully wipe out pan. Add 1 tablespoon oil and 1 tablespoon butter and fry 4 more pancakes. Repeat with remaining batter, wiping out pan and adding 1 tablespoon oil and 1 tablespoon butter before each batch.