In a deep saucepan, cover the potatoes with cold water and bring to the boil. Simmer for 25 to 30 minutes. Drain the potatoes, and as soon as you can handle them, cut them into quarters. Lay the potatoes in a single layer in a shallow baking pan.
Preheat the oven to 425°F.
Whisk together oil, rosemary, sugar, vinegar and pepper (if using). Drizzle over the potatoes so they are well-covered; sprinkle with salt. You can gently stir the potatoes a bit to distribute the sauce, but be careful not to break them. Bake for 20 to 22 minutes.