In a 5 qt pot bring water to a boil. Add in cauliflower florets and simmer until fork tender. Drain and set aside.
In a large skillet add in butter over medium heat. Once melted sprinkle with flour and whisk until flour is absorbed by butter. Working with ½ cup at a time, slowly pour in milk and mix with whisk. Once milk has started to thicken, add in the next ½ cup. Once all the milk is thickened into the flour/butter mixture season with nutmeg, dijon mustard,onion powder, salt and pepper stirring until combined. Sprinkle in 1 cup of the mozzarella cheese and ¾ cup of the sharp white cheddar and stir until combine and remove from heat.
Preheat oven to 350°F.
In a small ovenproof skillet or 9x9 baking dish arrange cauliflower in a single layer. Pour cheese mixture over cauliflower. Sprinkle with remaining cheese and panko. Bake on center rack for 20 minutes, or until bubbling.
Once cooked top with chopped parsley and serve warm. Enjoy!
