Season and flour the beef
Heat 1 tablespoon oil in a large stockpot / heavy bottom roaster over high heat
Add half of the beef to the pan and sear, flipping the meat once it has developed a good brown sear on the bottom, then repeating on multiple sides. Transfer the beef to a clean plate.
Add an additional one tablespoon of oil to the pot, add the remaining beef, sear, and transfer to a clean plate.
Reduce heat to medium-high and add the remaining tablespoon of oil to the stockpot.
Add onion and mushrooms (if used), and sauté for 5 minutes, stirring occasionally. Looking for a decent color on the onions and very little moisture from the mushrooms.
Add garlic and sauté for 1 minute, stirring occasionally.
Stir in the flour and cook for 1 minute, stirring frequently.
Pour in the Guinness and use a wooden spoon to scrape the bottom of the pan to lift the fond.
Stir in the beef stock, carrots, potatoes, celery, tomato paste, bay leaf, Worcestershire, dried thyme, and the cooked beef (along with any of its accumulated juices).
Cover the pot and simmer on low for 1 1/2 hours, occasionally stirring, until the beef is completely tender and the potatoes are soft.
Remove the bay leaf, taste, season, and balance the flavor.