Heat the olive oil in a large pot over medium heat.
Add the chicken and cook for 5 to 6 minutes until lightly browned. Transfer to a plate.
Add the onion, carrots, and celery to the pot. Cook for 4 to 5 minutes until softened.
Stir in the garlic and cook for 30 seconds until fragrant.
Pour in the chicken broth and add the oregano, thyme, salt, and black pepper. Bring to a boil.
Return the chicken to the pot, reduce the heat to a simmer, and cook for 12 to 15 minutes until the chicken is fully cooked.
Stir in the cooked rice and spinach, if using. Simmer for 2 to 3 minutes until the spinach wilts.
Remove the pot from the heat and stir in the fresh lemon juice.
Taste and adjust the seasoning with additional salt, pepper, or lemon juice if needed.
Garnish with fresh parsley and serve warm.