With the Instant Pot set to "Saute", heat 1 tablespoon of oil. Brown the chuck roast pieces on all sides, about 1 minute per side. Do this in batches and remove the beef once browned.
In the same pot, add onion, celery, and carrots. Saute until the onion becomes translucent, about 3 minutes.
Add garlic and cook for another 30 seconds until fragrant. Stir in tomato paste to coat the vegetables. Add beef stock or a small amount of water to deglaze the pot, scraping up any browned bits.
Add the beef back in along with potatoes, mushrooms, beef stock, bay leaves, salt, and pepper. Stir to combine.
Lock the Instant Pot lid, ensuring the pressure release is set to "Sealing".
Select the "Pressure Cook" function and set it to high pressure for 30 minutes.
Once done, allow for a 15-minute natural release, then manually release the remaining pressure.
Open the lid, add frozen peas (if using), and select the "Saute" function again. Let the stew come to a simmer. Prepare the cornstarch slurry by mixing 3½ tablespoons cornstarch with 3 tablespoons water.
Gradually add the slurry to the simmering stew, stirring until it thickens.
Portion the beef stew into bowls, garnish with chopped parsley, and serve hot!