Soak sliced potatoes in a bowl of water in the fridge for at least 30 minutes. Drain, rinse, and pat dry.
In a large bowl, toss together the potatoes, onions, and olive oil.
In a medium skillet, melt butter on high heat. Add potatoes, onions, salt and pepper.
Cover and fry for 5 minutes. Flip with a spatula and fry for another 5 minutes uncovered.
Flip the potatoes a final time, adding more salt and pepper. Fry for another 5 minutes. Enjoy!