Pat the meat cubes dry with paper towel to remove any excess blood or moisture.
Heat 2 Tbsp of the olive oil in a large skillet or heavy-based flat-bottomed casserole dish that has a lid and fry half the beef until golden and caramelized on all sides (it doesn't have to be cooked through). Repeat this by adding a second 2 Tbsp of olive oil and frying the remaining half of the meat. Remove and set aside.
In the same pan, heat the remaining 2 Tbsp of oil and fry the onions, carrots, and celery with the herbs until softened and just starting to go brown, about 4 minutes. Add the flour and cook this for about a minute until all the oil has been soaked up and it is a paste-like consistency.
Add the stout and deglaze the pan, scraping off any bits that may be stuck. Cook until most of the liquid has cooked off. Add the stock and continue to cook for another minute to thicken.
Add the tin of tomatoes along with the browned meat and mushrooms. Add the Worcestershire sauce and a good few pinches of salt and pepper.
You can then let this bubble away on your stovetop on very low heat, or you can braise in the oven at 180°C / 350°F. Keep it covered for the first 2 hours of cooking, then take off the lid for the last hour.
As the stew is cooking, make sure you give it a stir every 20 minutes or so to ensure nothing sticks and to adjust seasoning.