Trim the chuck roast of any excess fat. Pat it dry with paper towel, then season generously with salt and pepper.
Set the Instant Pot to saute on high heat. Add the oil.
When pressure cooker is hot, brown the roast on all sides, 4-5 minutes per side. Transfer meat to a plate and set aside.
Add the onion and cook until starting to brown, about 3-5 minutes. Add garlic, thyme, bay leaf, and broth. Scrape the bottom of the pot to deglaze. Return the meat to the pot and secure the lid.
Set the Instant Pot to manual mode at high pressure for 1 hour.
When cooking is complete, use a quick release. Double check that meat is cooked through. If not, cook longer on manual high pressure. If it is cooked through, add the potatoes, carrots, and parsnips, submerging them in cooking liquid and securing the lid of the pot.
Select manual high pressure for 5 minutes. Allow pressure to release naturally for 10 minutes, then release any remaining pressure. The vegetables should be fork-tender and the meat should be falling apart. If this is not the case, add additional time.