Position rack in upper 1/3 of oven, preheat broiler.
Bring cauliflower, 1 1/4 cup milk, and salt to boil in a large ovenproof skillet over medium-high head. Reduce heat, cover and simmer until cauliflower is tender, about 5 minutes.
Meanwhile combine the breadcrumbs, 1/4 cup cheese, and oil in a small bowl.
Whisk flour and the remaining 1/4 cup milk in another small bowl until smooth, stir the mixture into the pan and cook, stir until thickened, about 1 minutes. Stir in the remaining 1/2 cup cheese, chives, mustard, and pepper. Sprinkle breadcrumbs over the top. Broil in oven until the top is crispy (1-2 minutes).