best coleslaw recipe

1 medium head green cabbage, finely shredded - 2 large carrots, peeled and grated - 1/2 cup red onion, finely chopped (optional) - 1 cup mayonnaise - 2 tablespoons apple cider vinegar - 2 tablespoons granulated sugar - 1 tablespoon Dijon mustard - 1 teaspoon celery seeds (optional) - Salt and pepper to taste