Preheat oven to 375°F
In a large pot of boiling water, cook the cauliflower flowerets until tender but still firm. About 5 minutes. Drain and transfer the cauliflower to a 2 qt casserole dish.
In a medium sized sauce pan over medium heat, melt the butter. Once melted whisk in the flour. Cook for 1 minute or until you can smell a slightly nutty aroma. Slowly pour in the milk, whisking constantly. Bring to a boil, once boiling, reduce heat and continue stirring until thickened about 1 minute. Stir in the gruyere cheese, salt, pepper, and nutmeg. Remove from heat.
Pour the cheese sauce overtop of the cauliflower. Sprinkle the parmesan cheese overtop.
To make the topping: In a small bowl combine the melted butter and breadcrumbs. Sprinkle this mixture overtop the parmesan cheese.
Bake for 20-25 minutes or until the top is golden brown.
Remove from oven, top with fresh parsley if you'd like, and serve.