Put the pancetta and oil in a 12-inch heavy-based skillet over medium heat and cook, stirring frequently, until the pancetta is crisp, 6 to 8 minutes. With tongs, transfer the pancetta to a plate. Add the potatoes to the skillet and spread them into a single layer so a cut side faces down. Cook until the potatoes are golden brown on the bottom (the pan should sizzle but not smoke; adjust the heat if necessary), 5 to 8 minutes. Turn the potatoes with tongs so the other cut side faces down. Cook until that side is deep golden brown, another 5 to 8 minutes. Reduce the heat to medium low and cover tightly with a lid. Cook, tossing occasionally, until the potatoes are tender when pierced with a fork, 10 to 15 minutes more. If the potatoes are browning too much, reduce the heat to low.

While the potatoes cook, combine the garlic, lemon juice, and rosemary in a small bowl. When the potatoes are tender, add the garlic mixture, stirring to distribute it gently. Cook uncovered until the garlic mixture is heated through and fragrant, about 1 minute. Crumble the pancetta over the potatoes. Season to taste with salt and pepper and serve immediately.