Wash the potatoes. Then slice eacch potato in 1/4-1/8" slices. I use a large serving spoon to help prevent the knife from going all the way through. Just place a potato on the spoon and slice until the knife hits the spoon. Gently pull to separate the slices a little bit so you can season them between the slices.
Pat the potatoes as dry as you can so they will crisp in the oven.
Use 1/2 of the oil to rub the potatoes--being sure to get in between all the slices.
Stuff the slices with finely chopped fresh rosemary and salt.
Roast on a baking tray for 50 minutes. Then drizzle the potatoes with the remaining half of the oil, and roast for another 30-40 minutes or until crisped on the edges and fork tender in the center. Remember the timing may differ slightly depending on how large your potatoes are and how hot your oven runs.