Preheat the oven to 400 degrees F and position a rack in the top third of the oven.
Line a baking sheet with parchment paper. Place the potatoes and lemons on the sheet, drizzle with the oil and sprinkle with salt and pepper. Roast until tender and browned, about 1 hour, stirring the potatoes once or twice during cooking. Sprinkle with the garlic and roast 5 more minutes.