Preheat an oven to 375 or a grill over medium-high heat.
Wash and scrub the potatoes. Place on the cutting board and allow the potatoes to roll until they settle on their flattest surface.
Hold one of the potatoes, bridging your hand above the potato so that the knife can cut below. Start on one end of the potato and carefully slide your in cutting down, but not through the potato, leave about 1/2 inch to 1/4 inch of connection at the base. Move your knife about an 1/8 inch over and slice down again, leaving another 1/4 inch base connection. Repeat until you have an accordion of cuts across the potato.
Repeat this method with ALL of the potatoes. If they fall apart, no worries, just plan to make a thicker base.
Spread out a big square of foil, drizzle in the olive oil and stir in the garlic. Toss the ‘sliced into potatoes’ in the garlic-oil until well coated. If you can open up some of the slices to make sure the oil gets in the cracks that is great, too. Sprinkle with the salt and pepper.
Fold the foil around the potatoes creating a packet. Place the packet on a baking sheet in the hot oven or directly onto the hot grill. Roast for at least 30 minutes, then open up the packet. (Take care to not steam burn yourself. I state this warning from experience!) Roast for another 15-30 minutes until the edges of the potatoes have started to crisp slightly. Keep warm in the oven or serve immediately.