Set Instant Pot to Sauté mode. When hot, add oil and heat until shimmery.
Add chicken and sear 5 minutes per side until well-browned. Transfer to plate.
Pour water into pot to deglaze, scraping up browned bits with wooden spoon.
Whisk in au jus gravy mix until combined. Return chicken to pot.
Sprinkle ranch mix over chicken, then top with pepperoncini and butter pats.
Secure lid with vent in Sealing position. Set to Manual High Pressure for 12 minutes.
When cooking completes, quick release pressure carefully.
Remove chicken and shred into bite-sized pieces.
Skim fat from cooking liquid, return chicken to pot, and stir to coat.
Serve warm over mashed potatoes or preferred sides.
Slow Cooker Instructions: Place chicken in slow cooker, sprinkle with seasoning mixes, add peppers and butter. Cook on low 6 hours or high 2-3 hours. Shred and stir with cooking juices.