Season the chuck roast generously with salt and pepper, and let it sit for 15 minutes.
Press saute and add the oil to the Instant Pot liner. Once hot, sear the chuck roast, about 2-3 minutes per side. Shut off saute. Push beef to the side of the pot.
Add in mustard, parsley, garlic powder, onion powder, celery seed and beef broth. Stir it together and scrape up any browned bits off the bottom of the pot.
Switch the valve to sealing. Cook on high pressure for 50 minutes, with a 15-minute natural pressure release. Switch the valve to venting to release any remaining pressure after the 15 minutes.
Open the lid and transfer the meat to a plate, cover and set aside to rest.
Add in the potatoes and carrots and close the lid. Seat the valve to sealing. Cook on high-pressure for 2 minutes. Switch the valve to venting and release all pressure.
Open the lid and press the saute function.
Whisk together tapioca starch and coconut aminos in a small bowl to create a slurry. Add in 2 tablespoons of the slurry, and cook until the mixture is boiling. Add in more slurry if needed to get your desired thickness of gravy.
Shut off the saute function.
Slice or shred the pot roast. Serve hot with carrots, potatoes and gravy. Garnish with fresh thyme and parsley, if desired.