Build a campfire or heat the oven to 425 degrees F.
Make slices across each potato about 1/4-inch apart, making sure not to cut all the way through; the slices should remain joined at the bottom. To do this, you can line your work surface with 2 parallel chop sticks and place a potato between them so you will not be able to cut through completely.
Place a sage leave between each potato slice (tear the leaf in half if it is too large). Add a sprinkling of the garlic. Drizzle with olive oil and sprinkle with salt and pepper. Wrap each potato in 2 layers of heavy-duty foil. If using a campfire, bury the package in the coals. Make sure you don't put them in the dead center of the fire or you'll end up with potato ash. Cook until tender and crisp, 35 to 45 minutes. The length of time will depend on how hot the fire is. Alternatively, you can bake the potatoes in the oven until tender, about 45 minutes.
Drizzle lightly with more olive oil before serving.