Shred the potatoes on a box grater and transfer to a strainer set over a bowl to drain. Thinly slice the onion and add to the potatoes. Squeeze out as much excess moisture from the mixture as you can. Transfer the mixture to a large bowl and season well with salt. Add the eggs then the flour, stirring well with a wooden spoon. Form pancakes that are about 3 inches in diameter using your hands.
Put a skillet over medium heat and coat with some of the olive oil. Saute the pancakes in batches for 4 minutes, then flip, add 1 tablespoon of butter and cook for another 6 minutes until golden. Season lightly with salt as you remove the latkes from the skillet to a rimmed baking sheet and transfer to the oven to stay warm. Repeat with the remaining latkes, olive oil and butter.
Straining the potato and onion mixture allows for excess water to purge and makes crisper latkes.