Cook potatoes in boiling salted water to cover 12 to 14 minutes or until tender. Drain and let cool.
Cut potatoes into quarters, leaving skin on or peeled, if desired.
Heat olive oil and butter until hot; add onion, and remove when it starts to turn golden. Discard onion. Add potato, and fry 6 to 8 minutes or until golden on each side, turning once.
Remove potato from skillet; drain well on paper towels. Sprinkle with salt. Serve immediately.