Place the chicken breasts at the bottom of the crockpot. Add the chicken broth, diced onion, minced garlic, sliced carrots, sliced celery, thyme, oregano, bay leaf, salt, and pepper.
Cover and cook on low for 6-8 hours or on high for 4-5 hours, until the chicken is cooked through and tender.
Remove the chicken from the crockpot and shred it using two forks. Return the shredded chicken to the pot.
Stir in the frozen peas and baby spinach. Cook for an additional 15-20 minutes on high until the peas are heated through and the spinach is wilted.
Squeeze in the lemon juice before serving for a bright flavor. Remove the bay leaf before serving.