Preheat your oven to 425°F (218°C) and position rack in the middle.
Pat chicken breasts dry with paper towels to remove surface moisture. This step is critical for proper browning.
In a small bowl, combine salt, pepper, paprika, garlic powder, thyme, and cayenne (if using).
Brush chicken breasts with olive oil on both sides, then apply the spice mixture generously, ensuring even coverage.
Heat a large skillet over medium-high heat. Once hot, add the seasoned chicken breasts and sear for 2-3 minutes per side until golden brown (not fully cooked).
Remove chicken to a plate and set aside. Reduce oven temperature to 375°F (190°C).
In the same skillet, add butter and sauté onions, carrots, and celery for 4-5 minutes until onions become translucent.
Add minced garlic and cook for an additional 30 seconds until fragrant.
Stir in uncooked rice, coating it with the vegetable mixture and toasting slightly for 1-2 minutes.
Transfer the rice-vegetable mixture to the casserole dish, spreading evenly.
Pour chicken broth over the rice mixture.
Place the seared chicken breasts on top of the rice mixture, maintaining space between pieces.
Cover tightly with aluminum foil, ensuring a proper seal to trap steam.
Bake covered for 25 minutes.
Remove foil, add frozen peas (distributed evenly), and sprinkle with Parmesan cheese.
Return to oven, uncovered, and bake for an additional 15-20 minutes until chicken reaches 165°F (74°C) internal temperature and rice is fully cooked.
Allow to rest for 5-10 minutes before serving to redistribute juices.
Garnish with fresh chopped parsley before serving.