In a large Dutch oven, heat the vegetable oil over medium-high heat. Add the beef cubes in batches and brown them on all sides. Remove the beef from the pot and set aside.
In the same pot, add the diced onion and sauté for about 5 minutes until softened. Add the minced garlic and cook for an additional 1 minute.
Add the chopped carrots and celery, and cook for another 5 minutes, stirring occasionally.
Stir in the tomato paste and cook for 2 minutes. Then, return the browned beef to the pot.
Pour in the beef broth and Guinness stout. Add the dried thyme, bay leaves, salt, and black pepper. Bring the mixture to a simmer.
Reduce the heat to low, cover the pot, and let it simmer for about 2 hours, or until the beef is tender. Stir occasionally and adjust seasoning to taste.
If a thicker stew is desired, mix the flour with a little cold water to create a slurry, then stir it into the stew and simmer for an additional 10-15 minutes.
Remove the bay leaves before serving. Garnish with fresh parsley.