Heat oven to 400 degrees F.
In large bowl, toss together the cauliflower flowerets, olive oil, salt and pepper until cauliflower is well coated. Transfer to large rimmed baking sheet, at least 15 x 10 x 1 inches.
Roast cauliflower in 400 degree F oven, stirring occasionally, for 30 minutes or until the cauliflower is tender and evenly browned.
Near end of the roasting time, in medium-size saucepan, melt butter over medium heat. Sprinkle the flour over the butter in the saucepan; whisk until the flour and butter are blended and the mixture is slightly brown, about 3 minutes. Gradually whisk in the milk until smooth. Add salt, pepper and nutmeg; simmer, whisking occasionally, 3 minutes. Remove the saucepan from the heat. Whisk in the cheese and cayenne until the cheese is melted and well blended.
Transfer the cauliflower to a large serving platter. Spoon the sauce over the cauliflower. Sprinkle the parsley over the top as a garnish, if desired. Serve immediately. Makes 8 servings.