Remove the leaves from the cauliflower and cut off the excess stem. Use a paring knife to score around the core of the cauliflower and remove the core. Chop the core in half vertically then chop those into small pieces.
Turn the cauliflower over and cut into approximately 1/2" slabs to expose the inner stems. Use a paring knife to remove the stems from the florets. Chop the stems into smaller pieces if they are kind of thick. This will allow them to soften better when cooked. Keep the florets and stems separate.
Place the stems and core, along with a handful of florets, (around a cup), into a Dutch oven or large pot. Add 2 cups of water and 3 tablespoons of butter and bring to a boil over high heat. Reduce the heat to a simmer and place a steaming basket into the pot along with the remaining florets. Cover the pot and simmer for 10 to 12 minutes until the florets are tender.
Remove the steamer basket and place the florets in a bowl and reserve. Add the salt, pepper, mustard, nutmeg, and cayenne pepper to the pot and place the pot back on the stove and simmer for an additional 10 to 12 minutes for the stems and core to get well softened. Remove the pot from the heat and pour the contents into a stand blender. Add 1/2 cup of grated parmesan cheese and the cornstarch and water slurry and blend for 45 to 60 seconds until the sauce is smooth. Taste and adjust seasonings if desired. As an alternative, an immersion blender could be used to make the sauce smooth.
While the cauliflower is cooking, melt 1 tablespoon of butter in a skillet over medium high heat then stir in the panko breadcrumbs to coat. Continue to stir the crumbs around until they are a nice golden color. Pour the crumbs into a small bowl to cool. Once cool, add 1/2 cup grated parmesan cheese and stir to combine. Reserve this for final baking.
Preheat your oven to 400°F or 200°C.
Use an oven safe 8" by 8" dish or 9" round dish and spray with cooking spray. Add the florets and then pour in the creamy cauliflower sauce. Use a spatula to smooth. Sprinkle with the panko and parmesan breadcrumbs. Place in the oven on the middle rack and bake for 15 to 20 minutes until the casserole is bubbling around the edges. Let the dish cool for 15 or 20 minutes before serving. Garnish with chopped parsley, green onions, or chives.