Step 1

Preheat the oven to 425°F. Line a baking sheet with aluminum foil and coat it with cooking spray. Set aside.

Step 2

Lay down two wooden spoons or chopsticks and place a potato in-between them. Using a knife, thinly slice the potatoes about every ¼ inch. Take care to only cut down until you reach the wooden spoons (the spoons will prevent you from cutting all the way through). Transfer potatoes to the prepared baking sheet

Step 3

In a small bowl, combine the butter and olive oil. Brush this mixture all over the potatoes (you will not use it all) and season generously with salt and pepper. Transfer to the oven and roast for 40 minutes. Carefully remove the sheet tray from the oven and brush the rest of the butter and oil all over the potatoes, taking care to reach between the layers. Sprinkle potatoes with the rosemary, season once again with salt and pepper and return to the oven. Roast until the potatoes are fork tender, about another 20 minutes. Transfer to a serving platter and serve at once.