Place 1/2 head thinly sliced cabbage (about 1 lb.) and 1 cup grated carrot in a bowl. Whisk together 1/2 cup apple cider vinegar, 1/4 cup sugar, 1/4 cup vegetable oil, 2 Tbsp. Dijon mustard, 2 tsp. dry mustard, 1 tsp. celery seeds, 1 tsp. kosher salt, and 1/2 tsp. freshly ground black pepper in a saucepan until sugar dissolves; bring to a boil over medium-high heat. Pour over cabbage mixture; toss to coat. Serve immediately.