Set your Instant Pot to 'Sauté' mode and add the olive oil, heating it for about 1 minute.
Add the diced onion, garlic, carrots, and celery to the pot. Sauté for about 3-4 minutes, until the vegetables are softened.
Add the chicken breasts to the pot along with the rice, chicken broth, thyme, parsley, black pepper, and salt. Stir to combine.
Close the Instant Pot lid and ensure the valve is set to 'Sealing'. Select 'Manual' or 'Pressure Cook' and set the timer for 10 minutes.
Once cooking is complete, allow for a natural pressure release for 5 minutes, then carefully switch the valve to 'Venting' to release any remaining pressure.
Remove the chicken breasts from the pot and shred them using two forks, then return the shredded chicken to the soup.
Stir in the frozen peas and let them heat through for about 2 minutes.
Taste and adjust seasoning if necessary. If desired, add lemon juice before serving.