Preheat oven to 425 degrees. In a large pot of salted boiling water, cook potatoes until just tender, 10 to 12 minutes. Drain. Let potatoes dry 10 minutes, then spread in an even layer on a rimmed baking sheet.
Drizzle potatoes with oil, and dot with butter. Taste a potato for seasoning. Add more salt if necessary, and season with pepper.
Roast potatoes until beginning to turn golden, 30 to 35 minutes. Distribute onion evenly over potatoes. Reduce heat to 400 degrees, and roast until potatoes are golden and onions are tender, 25 to 30 minutes. Garnish with rosemary.