Place chicken thighs, onion, carrots, celery, garlic, oregano, thyme, salt, and pepper in a 6-quart slow cooker.
Pour chicken bone broth over the ingredients and stir to combine.
Cover and cook on low for 6 hours or high for 3-4 hours until chicken is tender.
Remove chicken from the crockpot, shred with two forks, and return to the pot.
Cook orzo separately in salted boiling water until al dente to ensure optimal texture, then drain and add to the crockpot.
Stir in heavy cream, lemon juice, lemon zest, and butter to emulsify the broth.
Fold in fresh baby spinach and allow it to wilt for 2-3 minutes in the residual heat.
Adjust seasoning with salt and pepper to taste, then garnish with fresh parsley and Parmesan cheese before serving.