Place a rimmed sheet pan in the oven and preheat oven to 400 degrees F.
Cut the potatoes in half or quarters and place in a large bowl. Peel and chop the onions in large pieces and add to the potatoes.
Add the oil to the bowl with the potatoes and onions and toss until the vegetables are well coated. Evenly sprinkle with the salt, pepper and rosemary and toss again to evenly distribute the seasonings.
Carefully pour the vegetables onto the sheet pan and spread out into 1 layer. At 15 minutes and again at 30 minutes, stir with a spatula to change sides of vegetables in contact with hot pan.
Roast in the oven 45 minutes to 1 hour or until potatoes are fork tender, brown and crisp. Remove the potatoes from the oven, season with salt to taste and serve hot.