Cook bacon in a large Dutch oven on stovetop over medium-high heat, until crisp. Remove bacon from pan, reserving 1 teaspoon drippings in pan. Set bacon aside.
Heavily salt and pepper the chicken. Add chicken to bacon drippings in pan; cook 6 minutes, browning on all sides.
Transfer chicken to the bacon, reserving any drippings in pan.
Add leek, carrot, and celery to drippings in pan; sauté 5 minutes. Stir in 1 cup stock, scraping pan to loosen browned bits. Add to leek mixture, the bacon and chicken mixture, remaining 5 cups stock, salt, pepper, and thyme sprigs to slow cooker. Cover and cook on LOW for 2 hours on stovetop.
The last hour, add sweet potatoes; cover and cook on LOW for 1 more hour or until potatoes are tender.
Discard thyme sprigs. Cut chicken into bite-size pieces (optional); add kale, stirring until spinach wilts.
Serve with fresh Parmesan cheese!