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.