Place the chicken breast in a slow cooker or a heavy-bottomed pot.
In a small bowl, whisk together the tomato puree, apple cider vinegar, smoked paprika, garlic powder, and half of the sea salt and black pepper.
Pour the sauce over the chicken and cook on low for 4-6 hours (or simmer on the stove for 45 minutes) until the meat is tender and easily pulls apart.
While the chicken cooks, prepare the slaw by whisking together the Greek yogurt, Dijon mustard, honey, and the remaining salt and pepper in a medium bowl.
Toss the shredded cabbage with the yogurt dressing until well coated and set aside in the refrigerator to chill.
Once the chicken is fully cooked, use two forks to shred the meat directly in the pot, stirring it into the remaining sauce to ensure every piece is coated.
Lightly toast the whole grain bun until golden and warm.
Pile the shredded chicken onto the bottom bun, top with a generous heap of the tangy slaw, and close the sandwich.