Boil potato slices in generously salted water until just tender, 6–8 minutes; drain and let steam-dry in the colander.
Melt 1 tbsp butter with 1 tbsp oil in a large skillet over medium heat; cook onions with a pinch of salt until soft and golden, 8–10 minutes. Transfer onions to a bowl.
Add remaining butter and oil to the skillet. Arrange potatoes in a single layer and cook, undisturbed, until deeply golden, 4–5 minutes per side. Work in batches if needed.
Return onions to the pan; toss gently with potatoes. Season with kosher salt and black pepper; splash in vinegar if using.
Sprinkle with chopped parsley and serve hot while crisp.