If desired, make 3/4-inch-deep, evenly distributed incisions in roast and push one slice of garlic into each.
In 6-quart pressure cooker, heat 3 tablespoons oil over high heat. Add meat and brown well on all sides lifting meat frequently with spatula to prevent sticking—about 2 to 3 minutes per side. Add extra oil as needed. Season each browned side with salt and pepper, be conservative if you're using canned stock or bouillon. Remove meat to platter and set aside.
In blackened oil remaining in cooker, sauté the chopped onion, carrot, celery, and parsnip for 3 minutes, stirring occasionally, scrape any browned bits from bottom of cooker. Add beef stock, bay leaves, and dried thyme.
Set rack or trivet in cooker and place meat on it, broad side down, so that as much surface as possible submerges.
Lock lid into place, heat to high pressure over high heat. Reduce heat to just enough to maintain high pressure and cook 60 minutes. Let pressure drop naturally for 15 minutes (see " Releasing Pressure"). Remove lid, opening it away from you, to allow any remaining steam to escape.
When pot roast is done, it should be easy to pierce with a fork. If it is not sufficiently tender, lock lid back in place and heat to high pressure for another 5 minutes, let pressure drop naturally.
When roast is fork-tender, transfer to platter, set aside in warm place. Remove rack from cooker. Place potatoes, whole onions, and parsnip chunks in cooker. Lock lid into place and return to high pressure over high heat. Reduce heat to just enough to maintain high pressure and cook 5 minutes.
Meanwhile, slice meat. Reduce pressure in cooker using quick-release (see " Releasing Pressure"). Remove lid, opening it away from you, to allow any remaining steam to escape. With slotted spoon, transfer vegetables to platter around meat, set aside in warm place.
Over high heat, gradually whisk flour-butter mixture into liquid in cooker. Cook, stirring constantly, until mixture thickens—3 to 4 minutes. Remove bay leaves and season to taste. Pour gravy over meat and vegetables on platter, sprinkle with fresh thyme, if desired, and serve immediately.