Preheat oven to 375 - 400°. Grease a casserole with butter (roughly 2-quart volume). Set aside.
Bring a large pot of water to a rolling boil and salt liberally. Drop in the cauliflower and boil for 3 - 4 minutes. While that's happening, fill a large bowl (not glass) with ice and cold water. When the cauliflower's time is up, drain it and immediately dump it into the cold water.It should have only just begun to soften in this time.
Meanwhile, heat 2 tablespoons of butter over medium heat in a saucepan until hot and sizzling, and start to warm the milk and cream (in the microwave or stovetop). Add the chili and stir for a minute or two before sprinkling in the flour. Stir to combine and cook for about 3 - 5 minutes, until the raw flour taste is gone.
Splash in the sherry and vigorously whisk it into the roux before pouring in the warm milk, slowly, while you continue to whisk. Season it with salt and bring to the gentlest of boil before reducing the heat and simmer about 5 minutes, until it's thick.
Shut off the heat and whisk in the grated Parmiginao. Fold the cauliflower in gently so it's as evenly coated as you can get it before transferring to the casserole dish.
Bake in the oven for about 30 - 40 minutes, until bubbly and golden brown. Let it sit out of the oven for 5 minutes or so before digging in.
The cauliflower can be blanched and shocked an hour or so in advance. I lay it out on some paper towel or microfiber towels to dry them out as much as possible.