Turn Instant Pot to Sauté setting and add 1 Tbsp. oil to pot.
Once oil is hot, add half of the stew beef and brown. Remove browned beef and set aside. Add remaining beef and brown (adding all at once will keep the meat from getting a good sear).
Add remaining Tbsp. oil. Once oil is hot, add onion and sauté for 4-5 minutes to soften. Add mushrooms and garlic and sauté for 2-3 minutes longer.
Add meat back into pan and sprinkle flour, salt and pepper over meat and veggies and stir to evenly coat.
Add Guinness to pot and stir to deglaze the pan, scraping any baked on bits off the bottom of the pot. Add beef stock, Dijon, tomato paste, thyme, Worcestershire, and bay leaves and stir to evenly distribute and mix well. Add potatoes, carrots, and peas and stir.
Put pressure cooker lid on and close to LOCK position. make sure vent is set to SEALING position.
Set Instant Pot to pressure cook for 30 minutes on HIGH.
Once cook time is complete, use a wooden spoon and kitchen towel to QUICK RELEASE pressure. Stir and season stew with salt and pepper to taste.
Sprinkle each serving with parsley to serve if desired. Store leftovers in a sealed container in the refrigerator for up to five days, reheating using the microwave or stovetop.