Make the chicken:

In the base of a slow cooker, whisk the apple cider, light brown sugar, mustard, hot sauce, Worcestershire sauce, paprika, salt and pepper until well-combined.

Add the chicken and onions, layering them evenly into the slow cooker.

Cover and cook on low until the chicken is fork-tender, about 5 hours.

Make the apple fennel slaw:

Place the fennel in a large colander, sprinkle 1½ teaspoons of the salt over the fennel and gently toss to distribute; let it sit for 30 minutes to draw out excess water.

Shake off any excess liquid and, using paper towels, dry the fennel, gently pressing to release excess water; transfer to a large bowl.

Add the apple and toss to combine.

In a small bowl, vigorously whisk the olive oil, apple cider vinegar, mayonnaise, honey, poppy seeds, ground mustard, celery salt, pepper and remaining teaspoon of kosher salt, until emulsified.

Pour the mixture over the fennel and apple, and toss to coat.

Cover and place in the refrigerator for 1 hour. Toss once more before serving.

Assemble:

Using tongs, remove the chicken from the cooking liquid and transfer to a medium-sized bowl.

Strain the cooking liquid through a fine-mesh strainer into another bowl; reserve 1 cup and discard the rest.

Transfer the reserved liquid to a medium-sized saucepan over medium-high heat along with the ketchup and stir to combine.

Meanwhile, whisk the cornstarch with 3 tablespoons of cold water, until it dissolves, to make a slurry, add to the saucepan and stir to combine.

Bring to a simmer and cook, stirring frequently, until the mixture thickens, about 10 minutes.

Using 2 forks, shred the chicken. Pour ¾ cup of the thickened sauce over the chicken and toss until well-coated.

Place a slice of the cheddar cheese on the bottom half of the pretzel bun. Top with about ½ cup of shredded chicken.

Top with some Apple Fennel Slaw and cover with the top half of the bun.