Step 1

Position rack in upper third of oven; preheat broiler.

Step 2

Bring cauliflower, 1 1/4 cups milk and salt to a boil in a large ovenproof skillet over medium-high heat. Reduce heat, cover and simmer until the cauliflower is tender, about 5 minutes.

Step 3

Meanwhile, combine 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, stirring, until thickened, about 1 minute. Stir in the remaining 1/2 cup cheese, chives, mustard and pepper. Sprinkle with the breadcrumb mixture. Broil until the top is crispy and beginning to brown, 1 to 2 minutes.