Peel the potatoes and cut them into thin strips, like french fries (a mandoline or chip cutter can be used). Soak the cut potatoes in a large bowl of water for about 30 minutes so they release most of their starch—this will give them a crispy exterior and tender interior. Remove from water and dry thoroughly with paper towels.

Heat a skillet over medium heat. Add the oil and heat it. Add all the potatoes to the skillet.

Fry for 3-4 minutes (the potatoes on the bottom should not yet brown at this stage) and add the butter.

After 1-2 minutes, when the potatoes on the bottom begin to brown, gently flip them using a wooden spoon, or better yet, with a skilled toss of the skillet that flips all the fries and lands them upside down.

Fry for about 5 more minutes and check the potatoes on the bottom again—flip only the pieces that have browned (try to flip only those). If other pieces haven't browned yet, fry for another minute and check again.

Chop the onion and add it to the skillet.

Continue frying the potatoes in the skillet, checking every 2-3 minutes and flipping the pieces as needed until all the potatoes are browned on all sides.

Transfer the potatoes directly to a large serving plate.

Only now season the potatoes with salt and pepper. At this point, you can also add another tablespoon or two of butter. Serve immediately to the table and enjoy the best fried potatoes in the world.