Lightly season chicken with salt and pepper. Set aside.
Set Instant Pot to saute on high. Add butter, oil, carrots and onion. Saute for 4-5 minutes, stirring occasionally, until soft and golden.
Add minced garlic, oregano, remaining salt & pepper and stir. Cook until fragrant, about 1 minute.
Add rinsed rice, then sliced mushrooms to pot.
Pour chicken broth into pot, to cover rice and vegetables (additional broth may be needed).
Place seasoned chicken thighs on top of the rice mixture. Do not stir.
Cover with the lid and set the valve to the sealing position. Cook on manual high pressure for 10 minutes.
When done, allow the pressure to release naturally for 10 minutes. Release remaining pressure manually.
After pressure has released, remove the lid. Remove chicken and shred into bite size pieces.
Stir in shredded chicken, chopped parsley and peas (if using).
Top with parmesan cheese and additional parsley if desired. Serve immediately.