In a large bowl, toss together the cabbage, broccoli, carrots, onion, and ½ cup sugar. In a small saucepan, combine the vinegar, salt, celery seed, 1 teaspoon white sugar, mustard and oil. Bring to a boil, and cook for 3 minutes. Cool completely, then pour over cabbage mixture, and toss to coat. Refrigerate overnight for best flavor.
Recipe by Columbine Kitchen at https://columbinekitchen.com/?p=757