1-2teaspoonscornstarch mixed with 1 tablespoon water
Brown the meat: Set the Instant Pot to sauté. Heat the oil, then brown the meat (season as it cooks). Once browned, sprinkle with the flour and toss well (make sure it doesn't burn on the bottom of the pot, it's OK if it sticks a little though).
Start the gravy: Stir the onions and garlic into the meat and cook for 1 minute. Add the tomato paste and cook for 1 more minute. Pour the Guinness over the meat, scratching the browned bits off the bottom of the pot (make sure you catch them all or you'll get the burn warning). Simmer for 5-10 minutes to cook off the alcohol.
Cook the stew: Switch off the instant pot. Add the beef stock to the stew. Stir in the vegetables. Close the instant pot and set the valve to "sealing". Press the "stew" button.
Finish: Once the pot is indicating it's done with cooking, do a natural pressure release for 10 minutes (just unplug your pot and leave it alone). Then manually release any remaining pressure and open the lid.
If you want to thicken the stew: Set the instant pot to sauté and stir in the cornstarch slurry. Simmer until thickened. Alternatively, you can blend 1 cup of the stew and stir it back in.