Scrub potatoes under cold water. Do not peel.
Parboil the potatoes in salted boiling water, about 15 minutes.
Drain the potatoes. Chill for several hours to overnight.
Peel the chilled potatoes then coarsely shred them.
Add salt and pepper to the shredded potatoes and toss.
Heat a large non-stick skillet over medium heat.
Once the skillet is warm, add 1/2 the butter and 1/2 the oil.
Lower the heat and add the onion. Cook until tranlucent, about 5 minutes.
Use a slotted spoon to remove the cooked onions, leaving the butter/oil.
Add 1/2 of the gratted potatoes to the pan and press down.
Add the cooked onions and the remaining potatoes and press down again.
Fry until the bottom is brown and crisp, about 10 minutes.
Place a large oiled plate over the skillet.
Carefully flip the skillet so that the potatoes remain on the plate.
Add the remaining butter and oil to the skillet and re-heat.
Carefully slide the potatoes back into the skillet, brown side up.
Fry until the potatoes are cooked through and the bottom is browned.