Peel the potatoes if desired, then slice them into approximately 1/4-inch-thick rounds.

Place the potato slices in a large bowl of cold water. Rinse them several times to remove excess surface starch.

Drain the potatoes thoroughly and spread them on paper towels. Pat completely dry to reduce splattering and encourage crisp browning.

Pour enough vegetable oil into a large skillet to generously coat the bottom. Heat over medium-high heat until the oil is hot and shimmering.

Carefully add the potatoes and sliced onion to the skillet, spreading them into as even a layer as possible.

Let the potatoes cook undisturbed for several minutes until the bottoms develop a golden crust. Gently turn sections with a spatula.

Continue cooking and turning every few minutes until most of the potatoes are browned on both sides and the onions are softened and caramelized.

Reduce the heat to medium, cover the skillet, and cook for another 2–4 minutes, stirring gently once or twice, until the potatoes are fork-tender.

Transfer the fried potatoes and onions to a paper towel-lined serving dish and allow excess oil to drain briefly.

Season with salt and black pepper while still hot and serve immediately.