Add potatoes to a large stockpot and cover with water, making sure the water reaches at least 1 inch above the potatoes. Boil over medium-high heat for approximately 15–20 minutes, or until easily pierced with a fork. Drain and let cool for a few minutes.

Preheat oven to 425°F and line a baking sheet with parchment paper. Place potatoes on the baking sheet and gently smash each one with the bottom of a drinking glass until about 1/2 inch thick. Avoid smashing them too thin so the centers stay creamy.

Mix olive oil, minced garlic, and chopped rosemary. Brush the mixture over the potatoes and season with salt and pepper.

Roast for 30 minutes, or until the edges are browned and slightly crisp.

Spoon a small amount of cranberry sauce over each potato. A little goes a long way and keeps the cranberry from overpowering the Brie and potato.

Garnish with fresh rosemary leaves, if using, and serve immediately while the Brie is warm and melted. The potatoes stay creamy in the center, so they are best served with a fork rather than as finger food.

To make ahead, boil the potatoes earlier in the day and refrigerate them after draining and cooling. Smash, season, roast, and add the toppings closer to serving time.