Meanwhile, whisk flour and 1/4 cup milk in a small bowl until smooth. Heat the remaining 3/4 cup milk in a saucepan over medium-low heat until steaming. Whisk in the flour mixture; cook, whisking, until thickened, 2 to 4 minutes. Remove from the heat; add cheese, salt, pepper and nutmeg (if using). Drizzle over the broccoli. Serve warm.