Ultimate Veggie Burgers with Sweet and Tangy Napa Slaw

Name: Ultimate Veggie Burgers with Sweet and Tangy Napa Slaw
Ingredients: 3 cups shredded napa cabbages; ½ teaspoon sugar , plus extra for seasoning; 1 teaspoon table salt; 1 Fuji, Gala, or Golden Delicious apple , peeled and shredded; 1 celery rib , sliced thin on bias; 1 tablespoon cider vinegar , divided; 2½ tablespoons vegetable oil , divided, plus extra as needed; ⅛ teaspoon pepper; 3 tablespoons mayonnaise; 1 tablespoon apple butter; 1 garlic clove , minced; 1 tablespoon minced fresh chives; 4 Ultimate Veggie Burgers patties (see note); 4 hamburger buns , toasted if desired