Using the fine side of a box grater or the fine grating disc on a food processor, grate all the potatoes. Squeeze out the excess liquid and transfer to a large strainer lined with a kitchen towel (not terrycloth). Gather the cloth, twist tightly and squeeze out as much liquid as possible. In a medium bowl, whisk the onion, eggs, flour, 1 1/2 tsp. salt and 1 tsp. pepper. Mix in the potatoes until well coated.
In a large skillet, heat 1/2 cup oil over medium-high. Working in batches, and adding more oil as needed, add the potato mixture to the skillet in rounded tablespoons. Flatten with a spatula and fry until deep golden brown, 2 to 3 minutes per side. Transfer the latkes to a rimmed baking sheet lined with a double layer of paper towels. Transfer to a platter and serve with the toppings.