Preheat the oven to 200°C (or 180°C for fan-assisted ovens). Position the oven rack in the middle for even cooking.

Place the peeled and cut potatoes in a large roasting pan. Add the diced butter and sliced garlic, tossing gently to ensure the potatoes are evenly coated. Sprinkle the French onion soup mix evenly over the potatoes. Pour the boiling water over the mixture and place the rosemary sprigs on top. Season with salt and freshly ground black pepper to taste.

Cover the roasting pan tightly with aluminum foil to retain moisture. Place the pan in the preheated oven and bake for 30 minutes. Midway through, stir the potatoes to ensure even cooking and absorption of flavors.

While the potatoes bake, heat the olive oil in a medium-sized frying pan over medium-low heat. Add the sliced onions and cook, stirring occasionally, for approximately 10 minutes until softened. Sprinkle the brown sugar over the onions and continue cooking for an additional 5 minutes, stirring occasionally, until the onions are caramelized. Remove from heat and set aside.

After 30 minutes of baking, remove the foil from the roasting pan. Add the caramelized onions, spreading them evenly around the potatoes. Return the pan to the oven and bake uncovered for an additional 5 to 10 minutes, or until the potatoes are tender and lightly golden.

Remove the roasting pan from the oven and discard the rosemary sprigs. Transfer the dish to a serving plate and garnish with additional fresh rosemary for a decorative touch. Serve warm.