Make the Dressing: In a medium saucepan, combine the sugar, vegetable oil, and apple cider vinegar. Heat the mixture over medium heat, stirring occasionally until it reaches a boil and the sugar has completely dissolved. Once the sugar is fully incorporated, take the saucepan off the heat. Stir in the celery seed, kosher salt, and black pepper. Let the dressing cool slightly while preparing the slaw.

Prepare the Vegetables: In a large mixing bowl, combine the thinly sliced onion, green bell pepper, and the coleslaw mix. Toss the vegetables together to evenly distribute the onion and bell pepper throughout the cabbage mix.

Combine the Slaw and Dressing: Once the dressing has cooled to room temperature, pour it over the coleslaw mixture. Toss everything together to ensure the dressing evenly coats all the vegetables. Seal the bowl with plastic wrap and place it in the refrigerator for a minimum of 1 hour to let the flavors blend together. For best results, let the slaw sit in the fridge for 2-4 hours before serving.

Serve: Just before serving, give the coleslaw a light toss to evenly distribute the dressing. Serve chilled alongside your favorite barbecue, sandwiches, or as a refreshing side dish for any meal.