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.