Place the cubed potatoes in a colander and rinse under cold running water to remove excess starch. Drain thoroughly and pat the pieces dry with a clean towel or paper towels.

Transfer the dried potato cubes to a large bowl. Drizzle with the cooking oil (or bacon fat) and toss until each piece is evenly coated.

Heat a non-stick skillet over medium heat and add the butter. Once the butter has melted, add the oiled potatoes, chopped onion, salt, and pepper. Stir to combine, then cover the skillet and cook undisturbed for about 10 minutes.

Remove the lid, increase the heat to medium-high, and continue cooking another 8–12 minutes. Flip or toss the potatoes occasionally with a spatula so they develop a deep golden, slightly crisp crust on most sides. Serve hot.