Place a skillet over medium heat. Add the bacon and cook until brown, then add in the onions and garlic. Sauté for 3-4 minutes to soften. (If using turkey bacon, cook in a nonstick skillet.) Pour the onions mixture into the crock of a large 6 quart slow cooker.
Place the chicken breast on top of the onions. Add the sliced potatoes, carrots, celery, chicken broth, thyme, 1 1/2 teaspoons salt, and 1/2 teaspoon black pepper.
Place the lid on the slow cooker. Turn on high and cook for 8-12 hours, stirring occasionally, until the potatoes are extremely soft. (Check the chicken after 6 hours.)
Shred the chicken and return to the soup. If desired, use an immersion blender to partially blend the soup for a creamier texture.
Serve hot, garnished with fresh parsley if desired.