Pat the beef dry with a couple of paper towels, then season well with salt and pepper.
Heat half the butter in a large, heavy-bottomed casserole pot on medium to high heat until it's almost smoking.
Brown the meat in batches about 3 minutes on each side without stirring. Put to one side.
Add the remaining butter and reduce heat to medium.
Add the onions and 1/2 teaspoon of salt and brown the onions (takes about 15 minutes)
Add flour and stir until onions are evenly coated and flour is lightly browned.
Add the broth continually stirring.
Stir in beer, thyme, bay, browned beef and any juices, and a little salt and pepper to taste.
Increase heat to bring to a full simmer.
Reduce heat to low, partially cover and leave cooking for 2-3 hours until beef is tender. Stir occasionally, loosening anything that sticks to the bottom of the pan.
Half an hour before the cooking ends add the mustard and sugar.
Remove the thyme and bay leaf.
Add a little salt and pepper to taste.