Cut the potatoes into roughly 1/2-inch cubes and place them in a bowl of cold water to remove surface starch. If using thin-skinned potatoes like Yukon Gold, peeling is optional. Drain and thoroughly pat dry with a clean kitchen towel.

Peel (if desired) and slice the onion into thin strips so it cooks evenly with the potatoes.

Heat a heavy skillet or cast-iron pan over medium heat and add the avocado oil. Warm the oil until it shimmers but does not smoke.

Carefully spread the diced potatoes into a single layer in the hot pan, taking care not to crowd them so they can crisp.

Let the potatoes cook undisturbed for about 3 minutes to form a golden crust, then stir. Continue cooking another 6–8 minutes, stirring every 2 minutes, until most sides are nicely browned.

Toss in the sliced onions and sprinkle with salt and black pepper. Stir to combine the potatoes and onions so they begin to mingle.

Lower the heat slightly and continue cooking about 4–6 minutes, stirring occasionally, until the onions are softened and lightly caramelized and the potatoes are cooked through.

Remove from heat and transfer to a serving dish. Serve warm as a breakfast side, brunch dish, or accompaniment to dinner.