Use a sharp knife to cut your potatoes into ⅛ th-inch thick slices, cutting most of the way through the potato. Take care to leave the bottom of the potato intact. Each potato should look like a closed “fan”.
Place potatoes on a sheet pan.
In a small bowl, whisk together the oil and butter. Brush oil mixture all over potatoes, making sure to baste between slices. Sprinkle with salt and pepper.
Bake at 375 for 50 to 60 minutes, until potatoes are tender and browned. Rest 5 minutes before serving. Your potato fans will open up during cooking.