Heat the oven to 200°F and set a rack over a rimmed sheet pan. Grate potatoes and onion on the large holes of a box grater, wrap in a clean towel and squeeze very firmly until little liquid comes out.
Mix the squeezed shreds with eggs, flour, salt and pepper just until evenly coated. Divide mentally into 16 equal portions and cook promptly.
Heat 2 tablespoons oil in a large heavy skillet over medium-high heat until shimmering. Add 4 portions and flatten each to about 3 inches wide and 1/4 inch thick, leaving space between them.
Cook for 3 to 4 minutes per side until deeply golden and the centers are tender with no wet egg mixture. Transfer to the prepared rack in the warm oven.
Repeat in 3 more batches, using 2 tablespoons fresh oil for each batch and adjusting the heat to prevent scorching. Serve 2 pancakes per person promptly.