Preheat oven to 450 degree F.
In a small bowl, whisk together the oil, the butter, and salt and pepper to taste.
Arrange the potatoes slices, not touching, in rows on an oiled baking sheet. Brush the potatoes with the oil mixture, and roast them in the upper 1/3 of a preheated oven, turning them over once with a spatula, for 18-22 minutes, or until they are golden and crisp.
Transfer the potatoes to paper towels to drain, sprinkle them with more coarse salt and pepper to taste (if desired), and serve them warm.