Remove onion skins and stems, and slice onions into medallions. Peel potatoes and slice into medallions, about 1/4 inch thick.
Toss medallions in oil in a large bowl. Toss carefully, trying not to let the onion rings separate. Place in single layer on a baking sheet. Sprinkle with salt and pepper. Cover with foil.
Place in cold oven. Heat oven to 400. Cook 30 minutes.