Heat a 10-inch heavy skillet over medium high heat.
Slice potatoes and onions very thin. Salt potatoes and onions. Add oil and butter to the pan. When the butter foams, add potatoes and onions to the skillet.
Place a dinner plate on top of potatoes and weight it with any heavy object: a sack of flour, heavy canned goods, etc. Let the potatoes and onions crust, 2 to 3 minutes, then turn, replace weights, and let them crust again. Keep turning the potatoes and onions over for about 20 minutes, until they are evenly golden and crusted.