Slice the potatoes thinly (~1/8 inch) without cutting all the way through.
Place the potatoes on a baking sheet or in a baking dish, stuff the sliced butter in between a few of the cuts in the potatoes. Disperse the garlic bulbs and season potatoes and garlic with salt and pepper and drizzle with olive oil.
Tent foil and roast in oven until potatoes are crispy on the top and tender in the middle, about 1 hour. Remove foil after the first 35 minutes.
Sprinkle the potatoes with fresh parsley and squeeze fresh lemon over top.