Heat olive oil in a large Dutch oven over medium-high heat. Add the beef cubes and sear until browned on all sides, then remove and set aside.
Lower the heat and add the diced onions, cooking until softened. Stir in garlic and tomato paste, cooking for another minute until fragrant.
Pour in the Guinness stout, scraping up any browned bits from the bottom of the pot. Simmer for 5 minutes to reduce slightly.
Return the beef to the pot and add the beef broth. Stir in carrots, potatoes, salt, and black pepper. Bring to a simmer, cover, and let cook for 2 hours on low heat.
For a thicker stew, mix the flour with a small amount of water to create a slurry, then stir it into the stew and cook for another 10 minutes.
Taste and adjust seasoning as needed. Garnish with fresh parsley before serving.