Season chicken with salt, pepper and oregano (optional).
In a skillet over medium heat, heat 1 tbsp oil and cook chicken pieces or 2-3 minutes per side, until no longer pink. Transfer to a plate and set aside.
Add the remaining 1 tbsp of oil. Add onion and cook for 2 minutes, until no longer translucent. Add garlic and mushrooms and cook for 5-6 minutes, until water from mushrooms has almost evaporated. Add peppers and rice. Cook for another 1-2 minutes, stirring frequently. Add chicken stock and spinach and stir to combine. Cover with a lid or foil and transfer to the oven.
Cook covered for 30 minutes. Remove lid/foil and add in chicken pieces. Add some water or chicken stock, if too dry. Bake uncovered for about another 10 minutes. (Chicken is cooked when the temperature of the meat is 165F).