Start the instant pot in SAUTE mode and heat it. Add oil, onions and garlic. Sauté for 2 minutes. Add carrots, celery, salt, pepper, thyme and broth. Stir well.
Place chicken breast on top and push it under the liquid. Set the instant pot on pressure cook mode at high pressure for 6 minutes.
When the instant pot beeps, let the pressure release naturally for 10 minutes, then open the lid. Remove the chicken breasts from the top and shred with two forks.
Stir in lemon juice and parsley. Taste and adjust seasonings.