How to Make It
Select SAUTÉ setting on a programmable pressure multicooker (such as Instant Pot). (Times, instructions, and settings may vary according to cooker brand or model.) Select HIGH temperature setting; add 1 tablespoon of the oil to cooker, and allow to preheat 2 to 3 minutes.
Season chicken pieces with salt and pepper. Add half of chicken to cooker; cook until browned, 3 to 4 minutes per side. Transfer chicken to a plate; add remaining 1 tablespoon oil and chicken to cooker, and cook until browned, 3 to 4 minutes per side. Transfer chicken to plate.
Add onion, celery, and carrots to cooker, and cook, stirring occasionally, until onion is translucent, 4 to 5 minutes. Add garlic to cooker, and cook until fragrant, about 1 minute. Press CANCEL.
Return chicken to cooker; add 4 cups of the stock. Cover cooker with lid, and lock in place. Turn steam release handle to SEALING position. Select MANUAL/PRESSURE COOK setting. Select HIGH pressure for 5 minutes. (It will take 10 to 12 minutes for cooker to come up to pressure before cooking begins.)
Carefully turn steam release handle to VENTING position, and let steam fully escape (float valve will drop). (This will take 4 to 6 minutes.) Press CANCEL.
Remove lid from cooker. Remove chicken from cooker; let rest 5 minutes, and shred.
Meanwhile, add noodles and remaining 4 cups stock to cooker. Select SAUTÉ and HIGH temperature setting; cover cooker with lid, and lock in place. Cook noodles and stock for 8 minutes. Carefully remove lid, and stir in lemon juice, parsley, and shredded chicken.