1 Sear the meat: Pat the pieces of beef dry with a paper towel, then season them on all sides with the salt and pepper.
Select the high “Sauté” setting on the electric pressure cooker and heat the butter or oil. Brown the beef in the butter in two batches, searing the pieces for 4 minutes per batch and flipping them halfway through cooking.
Don’t worry about trying to sear every side of the beef – you’re just trying to get some browning on the beef and some flavor developing in the pot. Use a pair of tongs to transfer the meat to a dish.
2 Cook the onions and celery: Add the onions and celery to the now-empty pressure cooker. Cook until the onions begin to soften and turn translucent, about 4 minutes. Stir in the tomato paste and cook for another minute.
3 Add the liquids to the pot: Pour in the Guinness. Use a stiff spatula to scrape up any browned bits from the bottom of the pot. Let it simmer for five minutes to reduce the beer a bit, then stir in the reserved beef, beef broth, and thyme.
4 Pressure cook the meat: Secure the lid on the pressure cooker and make sure it’s set to its “sealing” position. Cancel the cooking program, then select the Meat/Stew,” “Pressure Cook,” or “Manual” setting, and set the cooking time to 30 minutes at high pressure. (The pot will take about 10 minutes to come up to pressure before the actual cooking time begins.)
When the cooking program finishes, let the pressure release naturally for 10 minutes, then release the remaining pressure by moving the pressure vent to its “venting” position.
At this point, you may also leave the stew on the “Keep Warm” setting for up to 10 hours, before continuing with the rest of the recipe. The beef will become more tender the longer it sits.
5 Add the root vegetables and pressure cook: When the pressure has released, open the pot. (Optional: for a leaner stew, you can use a ladle or spoon to skim off some of the fat.) Stir in the carrots, parsnips, and root vegetables.
Put the lid back on the pressure cooker and make sure it’s set to its “sealing” position. Cancel the cooking program, then select the “Pressure Cook” or “Manual” setting and set the cooking time to 3 minutes at high pressure. (The pot will take about 10 minutes to come up to pressure before the actual cooking time begins.)
6 While the stew is finishing, mix the cornstarch and water together in a small bowl to make a slurry.
7 Finish the stew: When the cooking program ends, perform a quick pressure release by moving the pressure vent to its “venting” position. (It’s also ok if you need to leave the stew on the “Warm” setting for a little while before venting the pressure and serving, though the vegetables will eventually start to get mushy if left too long.)
Stir in the cornstarch slurry right away while stew is still bubbling. The residual heat will thicken the stew within a couple minutes.
8 Serve: Ladle the stew into bowls and serve it hot, with fresh parsley sprinkled on top.