Wash the potatoes and slice them about ΒΌ inch thick. Slice the onion to roughly the same thickness.
Heat the vegetable oil in a large frying pan over medium heat.
Once hot, add the potatoes in a single layer. Fry until golden on one side, flip, and brown the other side. Remove and repeat in batches.
After the final batch, add the butter and sliced onion to the pan.
Once the onions soften slightly, return all the potatoes and season with salt and pepper.
Gently fold everything together, then let the mixture sit for 3β5 minutes between folds so it can brown against the pan.
Continue until the onions begin to caramelize and brown, about 10 minutes after combining everything. Remove from the heat and serve.