In a large pot or frying pan with a tight-fitting lid, heat the oil and 1 tablespoon of the butter over medium heat. Add the chicken and season liberally with salt and pepper.
Cook the chicken for about 5 minutes, or until cooked through. Transfer the chicken to a plate and set it aside.
Melt the remaining tablespoon of butter in the pot. Add the onion, celery, and bell pepper. Sauté for 4-5 minutes until softened.
Add the garlic, paprika, oregano, thyme, cayenne pepper, and crushed red pepper flakes. Cook for another minute until fragrant.
Stir in the rice and broth. Increase heat to high and bring mixture to a boil. Reduce heat to low and cover. Cook for 15 minutes, until the rice has absorbed most of the liquid.
Return the chicken to the pan. Cover and cook for another 5 minutes, until all of the liquid has been absorbed into the rice and chicken has warmed through.
Serve warm, garnished with cilantro if using.