Preheat oven to 400 degrees. Split the potatoes in half lengthwise, and cut the onions in half crosswise.
Pour the oil on an aluminum jelly roll pan. Sprinkle the salt over the potatoes and onions, and place them cut side down in one layer on the oiled pan. Place the pan on the bottom rack of the oven (or, if you want the underside of your vegetables very crusty, on the oven floor). Bake for 40 minutes, until the potatoes and onions are tender when pierced with a fork, lightly browned on top, and dark brown on the underside.
Set the potatoes and onions aide to rest for 10 minutes, lift with a spatula and serve.
2000, 2008, 2009