Add the onion, carrot, and celery to the bottom of the slow cooker. Pour in the chicken broth. Place the turkey tenderloins on top. Close the Instant Pot and cook on high pressure for 8 minutes. Let naturally release for 10 minutes.
To make gravy: Turn the Instant Pot back to Saute mode. In a cup, combine the cornstarch and cold water. Use a whisk to stir the cornstarch mixture into the chicken broth. Whisk until smooth and continue to cook until the gravy is thick and bubbling., Season with salt and pepper as needed. Slice the tenderloin and drizzle with gravy to serve.