Season the cubed beef with salt and pepper.
Add 2 tablespoons oil to a large 7-quart Dutch Oven and set it over high heat.
Once oil is hot, add the beef in a couple of batches and sear on all sides.
Remove beef from the Dutch Oven and set aside.
Add remaining olive oil to the pot. Stir in the garlic and onions and cook for 2 to 3 minutes, or until softened.
Add the carrots, celery, potatoes, and mushrooms. Cook for a further 3 minutes. Add more oil, if needed.
Sprinkle the flour over the veggies and mix well to coat. Cook for 2 minutes.
Add the Guinness beer and, using a wooden spoon, scrape up any browned bits from the bottom of the pot.
Return the beef to the pot; stir in the tomato paste, beef broth, bay leaf, thyme, and brown sugar.
Bring to a boil and then reduce heat to LOW; cover and allow to simmer for 1 1/2 hours with the lid on.
Remove lid and cook for 30 more minutes.
In a small cup or bowl combine water and cornstarch; whisk to combine.
Add cornstarch slurry to the stew and stir to incorporate.
Cook for 3 more minutes, or until thickened.
Remove stew from heat; taste and adjust salt and seasonings, if required.
Remove bay leaf and garnish stew with fresh chopped parsley.