Set your brie on a parchment-lined baking sheet or in a small oven-safe dish. I like to leave the rind on since it holds the cheese together as it melts, but you can trim a thin layer off the top if you want a gooier center. Let it sit at room temperature for about 10 minutes before baking so it warms evenly.
In a small saucepan over medium heat, combine the fresh cranberries, honey, and rosemary. Cook, stirring occasionally, for about 5 to 7 minutes until the cranberries soften and start to burst. You'll know it's ready when the mixture looks glossy and slightly syrupy.
While the cranberries simmer, toss the chopped walnuts in a dry skillet over medium heat for 2 to 3 minutes, stirring often. This step is optional but I always do it since it brings out a much richer, toastier flavor than raw walnuts.
Preheat your oven to 350°F. Spoon the warm cranberry mixture evenly over the top of the brie, letting a little drip down the sides. This little glaze is what makes the finished dish look so festive.
Bake the brie for 10 to 12 minutes, or until it feels soft when you gently press the side with a spoon. Keep an eye on it near the end since ovens vary and brie can go from perfectly melty to a puddle fast.
Remove the brie from the oven and drizzle with a little extra honey. Scatter the toasted walnuts and pistachios on top, then let it rest for 2 to 3 minutes before serving so it firms up just slightly.