Preheat oven to 425°F. Line a baking sheet or roasting pan with foil.
Make cuts every 1/8 to 1/4 inch in each potato, slicing deeply but not all the way through, to create a fanlike appearance. Brush the potatoes with 3 tablespoons oil, taking care to coat the cut surfaces well. Season with pepper and salt. Place the potatoes on the prepared pan. Roast until golden brown and slightly crispy, 1 to 1 1/4 hours.
Meanwhile, whisk the remaining 3 tablespoons oil, lime juice, fish sauce and honey (or maple syrup) in a small bowl. Add lime zest, cilantro, peanuts, chiles and garlic; mix well. Serve the potatoes drizzled with the dressing.