Place chicken breasts in bottom of crockpot.
Add chicken broth, diced onion, sliced carrots, sliced celery, minced garlic, dried thyme, dried rosemary, black pepper, and salt to crockpot.
Stir to combine ingredients.
Cover and cook 4 hours on high or 8 hours on low, until chicken is cooked through and vegetables are tender.
Remove chicken and shred with two forks. Return shredded chicken to crockpot.
Add lemon zest, lemon juice, and spinach to soup. Stir well to mix flavors until spinach is wilted.
Taste and adjust seasoning as necessary.
If using, add cauliflower rice or zucchini noodles to soap. Cook for additional 20 minutes until tender.
Serve hot and garnish with slice of lemon