1. Pat the beef cubes dry and toss them with the flour in a large bowl until well coated.

2. Heat the tallow in a large Dutch oven or heavy-bottomed pot over medium-high heat. Working in batches if necessary, sear the beef on all sides until deeply browned, about 2-3 minutes per side. Transfer the seared beef to a plate.

3. Reduce the heat to medium and add the onions to the pot. Cook, stirring occasionally, until softened and translucent, about 5 minutes. Add the garlic and cook for an additional minute, until fragrant.

4. Pour in the Guinness and use a wooden spoon to scrape up any browned bits from the bottom of the pot. Add the beef broth, bay leaves, and thyme. Return the seared beef and any accumulated juices to the pot.

5. Bring the stew to a boil, then reduce the heat to low, cover, and simmer for 1 hour, stirring occasionally.

6. Add the carrots and potatoes to the pot and continue simmering, covered, for an additional 1 hour, or until the beef is very tender and the vegetables are cooked through.

7. Season the stew with salt and pepper to taste. Discard the bay leaves before serving.