"Your Thanksgiving vegetables just got an upgrade...."
(1/2 stick) unsalted butter, plus more for pan
medium head cauliflower (about 2 pounds), cored and sliced 1/4-inch thick
large head broccoli (about 1 1/2 pounds), trimmed and sliced 1/4-inch thick
all-purpose flour
whole milk
freshly grated nutmeg
Gruyère, grated (about 2 cups)
Kosher salt and freshly ground black pepper