Wash and dry the potatoes. To chop them, place a potato lengthwise between two chopsticks (or wooden spoons). Use a sharp knife to cut slices across the potato, about 1/8-inch apart.
The chopsticks/spoons stop you from cutting all the way through the potato. Just be careful at the ends, as they are more fragile and may still fall apart if cut too close to the end.
Transfer all potatoes to either a lined baking sheet or into the basket of your air fryer. Brush the sliced tops with oil and add slices of garlic and rosemary leaves between some potato slices.
For oven baked, bake the potatoes for around 60 minutes, basting them with more oil halfway through. The potatoes are ready when they are crispy and brown. Sprinkle with salt and pepper.