1. Preheat oven to 350 degrees Fahrenheit. Cook potatoes until just tender (approximately 25 minutes).
2. Meanwhile, spread onion and oil on the bottom of a nine- by 13-inch pan and cover pan with aluminum foil. Place pan in oven.
3. When potatoes are cooked, mix into onions and season with salt and pepper.
4. Place, covered, into oven for 10 minutes. Uncover and bake for another 45 minutes to an hour, tossing every 10 minutes until potatoes are lightly browned.