Add the broccoli florets to a large saucepan and cover with boiling water. Bring to a boil, then reduce to a simmer. Simmer the florets for 5-7 minutes or until just tender. Drain completely and set aside to steam dry.
Preheat the oven to 400 degrees Fahrenheit. Melt the butter in a saucepan over a low/medium heat. Add the cream cheese, rosemary, garlic powder and nutmeg. Stir well to combine, melting the cream cheese into the butter until smooth.
Add the stock to the saucepan with the cream cheese mixture. Season to taste and stir well to combine. Bring the mixture up to a gentle simmer for a few minutes.
Roughly chop the brie, removing any tough outer rind. Add the grated cheddar cheese, a tablespoon of grated Parmesan, and the chopped brie to the sauce. Mix well to combine over a low heat until the cheese is melted. Sprinkle over the xanthan gum and whisk well until you have a thick and creamy sauce.
Select a medium-sized oven dish. Arrange the part-cooked broccoli in an even layer in the base of the oven dish. Scatter the dried cranberries over the broccoli florets.
Spoon the cheese sauce over the broccoli and cranberries. Scatter the remaining grated Parmesan evenly over the top. Transfer the dish to the oven to bake for 15 minutes.
Roughly chop the walnuts. Remove the bake from the oven. Scatter over the chopped walnuts. Return to the oven for a further 5 minutes to toast the walnuts. The bake should be bubbling and golden. Serve hot with your Keto Christmas mains.