In a medium saucepan, cover the potatoes with an inch of water and season with a big pinch of salt. Bring the water to a boil and cook until the potatoes are just tender when pierced with a fork, about 5 minutes. Drain the potatoes.
Meanwhile, cook the onions. In a No.8 (10-inch) or No.10 (12-inch) cast iron skillet, heat the vegetable oil (or other fat) over medium heat. Add the onion and cook, stirring frequently, until golden brown, 3 to 4 minutes.
Add the potatoes in a single layer and season with salt and pepper. Cook, stirring every couple of minutes, until the potatoes are crisp and golden brown all over, 6 to 8 minutes. Season to taste with salt and pepper and serve immediately for maximum crispiness.
Field Notes
