Season beef chuck pot roast on both sides with salt and pepper.
Turn Instant Pot pressure cooker to Saute setting and allow to get hot. Add the oil and sear both sides of the beef chuck pot roast until browned, about 3 minutes per side. Remove from pot and set aside momentarily.
Add the onion to the pressure cooker, top with browned meat and pour in the beef broth and pepperoncini juice. Sprinkle the ranch dressing seasoning over and top with butter slices. Scatter 8 whole pepperoncinis over all. Close lid and vent.
Cook on High pressure for 60 minutes, followed by a 15 minute natural release. Then, switch the release valve to the venting position. Remove lid once steam has stopped coming out.
Shred meat with forks and slice pepperoncinis or leave whole. Serve over mashed potatoes or noodles of choice.
Adding butter to the roast is for flavor only and is optional