Roast the cauliflower florets by tossing them in light oil, placing them on an oiled, rimmed baking tray, and baking them in the oven, heated to 425°F.
Bake until the cauliflower is golden brown and beginning to crisp around the edges.
Turn over the cauliflower halfway through and continue to bake until the cauliflower is beginning to brown around the edges and offers no resistance when pierced with a fork, about 20 minutes.
Oil a 9 x 13 glass baking dish generously with butter.
Heat the oven to 325°F
Mix the half and half, heavy cream, egg white powder together in a large mixing bowl.
Add the mustard, parsley, seasoned salt, black pepper, and cayenne pepper and mix well.
Stir in the cheeses and the roasted cauliflower.
Pour the mixture into the prepared baking dish and cover with tin foil. Bake in the preheated oven for 20 minutes.
Remove the foil, top the mac and cheese with the crumbs and bake uncovered for another 15-20 minutes until the casserole is set and the crumbs are crisp and golden brown. Allow the mac and cheese to cool ten minutes before serving.