In a large saucepan combine cauliflower, 1 1/4 cups of the milk, and the salt. Bring to boiling; reduce heat. Simmer, covered, for 6 to 8 minutes or until cauliflower is crisp-tender.
Meanwhile, in a small bowl whisk together flour and the remaining 1/4 cup milk until smooth. Stir flour mixture into cauliflower mixture. Cook and stir until thickened. Stir in cheese, mustard, and pepper until cheese is melted. Transfer to a serving dish. If desired, sprinkle with chives.