Peel potatoes and cut into 1/8″ slices, then pat dry with paper towels. Heat oil in 1-” skillet over medium heat until hot.
Using half of the potatoes, quickly arrange them in skillet, slightly over lapping. Sprinkle half of the onion, salt and pepper on top, then cover with remaining potatoes. Sprinkle with remaining onion, salt and pepper.
Cook covered over low heat for 15 minutes without disturbing. Press slices firmly but gently with back of spatula. Cook covered, pressing potatoes occasionally, until edges are brown and crisp, about 15 minutes.
Let stand 3 minutes. Invert onto large plate and cut into wedges.