Warm the vegetable oil in a large soup pot over medium-high heat. Add the onions and cook, stirring frequently, for 3 minutes until softened.
Stir in the garlic and cook for 30 seconds. Sprinkle with flour and stir to coat.
Add the potatoes and carrots. Pour in the beef-style broth, Guinness, tomato paste, Worcestershire sauce, salt, thyme, and black pepper. Stir to combine. Simmer. Bring just to a boil then reduce the heat to low. Cover and simmer for 20 minutes until the vegetables are fork-tender.
Remove the lid and cook for an additional 15 minutes, stirring occasionally, until the broth has thickened slightly. Adjust seasoning to taste.
Ladle into bowls, sprinkle with fresh parsley, and serve hot.