Slice the bottom off each potato and cut crosswise at 1/8-inch intervals, cutting to within 1/4 inch of the bottom. (You can place the potato on a large spoon so the edges of the spoon prevent you from slicing completely through the potato.)
Place the potatoes in a large bowl, add the olive oil, salt and pepper and 1 tablespoon of the rosemary and toss well, working the salt mixture in between the slices. Arrange the potatoes on a sheet pan cut side up.
Bake the potatoes for 30 minutes, until they are tender and golden and crisp on top