In a large skillet with a tight-fitting lid, heat the oil and 1 tablespoon of the butter over medium heat. Add the chicken and season it 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.
To the same skillet, melt the remaining butter. Add the onion and celery and cook for 3-4 minutes until softened.
Add the garlic, thyme, rosemary, 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 skillet. Stir in the peas and lemon juice. Cover and cook for another 5 minutes, until all of the liquid has been absorbed into the rice and the peas and chicken have warmed through.
Serve warm, garnished with parsley if using.
Leftovers can be stored in the refrigerator for up to five days or in the freezer for up to three months.