Peel and dice the onions and carrots. Dice the celery, cut the broccoli into small florets, and grate the garlic.
Add the olive oil to the instant pot, and turn on the saute setting. Add carrots, celery and onions to the pot, and saute for a few minutes while stirring.
Then, add the broccoli, garlic, salt, pepper and Italian seasoning to the pot, and saute for another minute. Turn off the instant pot.
Rinse the brown rice in cold water, and add it to the instant pot along with chicken broth. Stir to combine the ingredients thoroughly, then use a spoon or potato masher to push the ingredients down. Scrape down any rice hanging on the edges of the pot.
Finally, add the chicken breasts on top. Don't stir. Cover the instant pot with a lid, turn the valve to sealing, and turn on the MANUAL or PRESSURE COOK setting on high. Cook for 20 minutes.
After the IP stops cooking, it will start to naturally release the pressure. Allow it to do this for 12 minutes. The dish will finish cooking during this time, so don't skip this step. After 12 minutes, turn the valve to do a quick release of any remaining pressure.
Remove the lid, and use tongs to pick up the chicken breasts. Dice or shred the chicken on a cutting board, then transfer it back to the IP. Add the frozen peas to the dish. Stir to combine everything thoroughly. The peas will thaw and cook in the rice within minutes.
Serve the dish immediately, optionally topped with parsley, scallions, Parmesan or lemon juice.