Adjust oven rack to lower-middle position and preheat oven to 160°C | 325°F. Season beef generously with salt and pepper.
Heat oil in dutch oven or a heavy based, oven-proof pot over medium-high heat. Sear beef in batches of three or four until browned all over. (Use more oil where needed.) Transfer to a warm plate.
Add onion and garlic to the pan juices; sauté until soft and transparent. Add the carrots, celery and potatoes, and cook for a further 2 minutes. Stir the flour into the vegetables in the pan, coating them evenly. Cook, stirring occasionally, for a further 2 minutes.
Pour in the Guinness; mix well to dissolve flour, then add the tomato paste, brown sugar, broth and thyme, scraping up any browned bits on the bottom of the pot with a wooden spoon. Bring to a simmer and cook until slightly thickened, (about 5 minutes).
Add the beef back into the pot along with any juices released.
Partially cover the pot; transfer to the oven and bake for 2 1/2 - 3 hours (stir stew twice through cooking).
Carefully remove from the oven, and season with salt and pepper, if desired, to your tastes. Garnish with parsley and serve with mashed potatoes.