Cauliflower Gratin with Manchego and Almond Sauce

half-and-half

unsalted butter

all-purpose flour

whole milk

freshly grated nutmeg

Salt and freshly ground pepper

head of cauliflower, cut into 1 1/2-inch florets

whole roasted almonds with skin, plus 2 tablespoons coarsely chopped almonds (3 ounces)

plus 2 tablespoons finely shredded Manchego or other mildly nutty semi-aged sheep's- or cow's-milk cheese (3 1/2 ounces), such as Gouda

medium onion, finely chopped

Pimentón de la Vera (smoked Spanish paprika)