Heat a large, heavy skillet over medium-high heat. Then add the oil and butter.
Lightly salt the sliced potatoes and onions. Add them to the heated pan (butter should be melted and foamy).
Put a heat-safe dinner plate on top of the potatoes and onions and then place a heavy object on top of the plate. Let the potatoes and onions get a nice crust, about 2-3 minutes, then turn them and replace the plate/weights to let the other side crust. Keep turning the potatoes and onions for about 20 minutes, until they are all golden and crusted.