Preheat the oven to 375 degrees. Coat a small baking dish with cooking spray.
Cook the cauliflower in salted boiling water until tender, about 3 minutes. Remove the florets from the water immediately and set aside until needed.
Heat two tablespoons of butter in a saucepan over medium heat.
Add the flour and cook, stirring constantly, for 2 minutes.
Add the hot milk and whisk constantly for a few minutes until boiling and thickened. Remove from the heat.
Add 1/2 cup of the gruyere along with the parmesan cheese. Whisk until well combined. Add the nutmeg then season with sea salt and freshly cracked pepper, to taste.
Pour 1/3 of the cheese sauce in the bottom of the small baking dish. Place the drained cauliflower on top then spread the rest of the cheese sauce on top of the florets.
Mix the panko crumbs with the remaining 1/4 cup of gruyere cheese until well combined.
Sprinkle evenly over the top of the cauliflower. Melt the remaining 2 tablespoons of butter and evenly drizzle on top of the panko crumbs.
Bake for 25-30 minutes, or until the top is golden brown. Top with chopped parsley, if desired. Serve hot or at room temperature. Enjoy!