Heat oil in a 6-qt instant pot using SAUTE function until shimmery. Add beef cubes to instant pot and season with salt and pepper. Sear beef cubes, turning occasionally until golden brown.
Add onions to beef and saute 3-4 minutes until onions begin to soften. Add tomato paste, brown sugar, and garlic and saute 1 additional minute until fragrant. Turn off SAUTE function.
Pour Guinness and beef broth into instant pot. Add bay leaf, thyme sprigs, potatoes, and carrots. Secure lid in place on instant pot. Cook on high pressure 35 minutes.
Let pressure release naturally 10 minutes, then release any remaining pressure by moving vent open. Unscrew lid and discard bay leaf and thyme sprigs.
Ladle out 1/4 cup beef liquid into a small bowl and rapidly whisk with flour until smooth. Pour flour mixture into beef stew and stir until beef stew is thickened, about 1-2 minutes.
Optionally, you can let beef stew sit in instant pot on WARM setting up to 2 hours. When ready to serve, ladle beef stew into bowls and top with parsley. Serve warm and enjoy!