Dry beef using paper towels. Toss with flour, salt and pepper until well coated.
Heat the oil in a deep skillet or Dutch oven over medium-high heat. Add the beef in batches, and brown on all sides. Remove beef from pot. Add 1 T. oil and the onions. Cook the onions until golden brown. Add 2-3 T flour to onions and stir to coat.
Return beef to pan and immediately add Stout and beef stock. Stir the tomato paste into a small amount of stock to dilute; pour into the pan and stir to blend. Reduce the heat to medium and add carrots and thyme, Worcestershire sauce and brown sugar.
Reduce heat to low, and simmer for 2 to 3 hours, stirring occasionally.
With about one hour of cooking time remaining, saute the mushrooms in butter until browned and liquid starts to sweat out. Add to stew.
Taste and add more salt and pepper to taste before serving.
Serve over colcannon (mashed potatoes with leeks and cabbage).