Sprinkle chicken with salt and place breast side down in the slow cooker. Add halved lemon, bay leaves, and peppercorns. Cover nearly with water, leaving an inch at the top. Cook for 7+ hours on low.
Transfer chicken to a tray and keep the liquid. Discard herbs and peppercorns. Remove chicken skin and bones, shred the meat, and set aside covered to keep warm.
In a large pot, sauté onions, celery and carrots. After a few minutes, reduce heat, add garlic and cook for 2 more minutes. Pour in the slow cooker broth and season with salt, pepper, and flavourings. Squeeze in leftover lemon juice.
Simmer until vegetables are soft enough to blend with a hand mixer. Be careful with hot liquid and allow to cool if needed before blending.
Add shredded chicken to the soup and heat through. Season further if needed and add fresh parsley before serving.
