In your slow cooker, whisk together the honey and whole grain Dijon mustard until smooth and well blended. Add in 1/4 cup of water to help create a saucy consistency.

Carefully place the boneless, skinless chicken thighs into the slow cooker, ensuring each piece is thoroughly coated with the honey mustard sauce.

Cover the slow cooker and set it to low heat, cooking the chicken for 4 to 4½ hours until fork-tender and shreds easily.

Mix 2 tablespoons of water with cornstarch in a small bowl to create a slurry. Stir this slurry into the slow cooker and cook on low for an additional 20 minutes.

Once thickened, use two forks to shred the tender chicken directly in the slow cooker and mix it back into the honey mustard sauce.

While the chicken cooks, combine sliced red cabbage, olive oil, apple cider vinegar, salt, and pepper in a bowl. Toss everything together until coated.

To assemble your sandwich, pile shredded chicken onto soft brioche buns and top with prepared slaw.