Prepare all ingredients and preheat oven to 350 degrees.
In a 9x13 baking dish pour just enough olive oil to lightly cover the bottom of the dish. Add potatoes, season with salt and pepper and toss to thinly coat. This will prevent them from burning and browning too quickly. Cook uncovered for 15 minutes.
Remove from oven and toss with onion and garlic. All pieces should be lightly coated. Return to oven for an additional 15 minutes.
Remove from oven and toss in rosemary. Return to oven for 15 minutes or until potatoes are fully cooked. Serve hot.