Cut off the bottom (lengthwise) of each potato to create a flat surface for the potato to rest on. With a sharp knife, carefully cut vertical slits into each potato about every 1/8- inch without cutting all the way through the potato.
Rinse the potato under running water and fan out the slits – be sure to rinse inside. Place potato on a microwave safe plate and microwave for 5 minutes. Flip potatoes over and microwave for an additional 5 minutes.
Once out of the microwave, place garlic and sliced parmesan into the slits of each potato. Brush potatoes with olive oil all over and season with salt and pepper. Sprinkle the grated parmesan on top.
Place potatoes on a piece of aluminum foil and bring the edges up around the potato, forming an enclosed pouch. Cook on a gas grill on medium-high heat until they are crispy on the outside but still soft on the inside, 30-40 minutes. If desired, add more cheese 4-6 minutes before done cooking.
Remove from grill, remove aluminum and let cool, then serve while still warm.