Cut the beef stew meat into about 3-inch cubes. Sprinkle with pepper, salt and flour.

Melt the butter in a large casserole over medium-high heat. Brown the meat in portions on all sides. Remove the meat from the pan and set aside.

In the same pan, add the onions and cook over low heat until soft and golden brown, about 10 minutes.

Add the mustard to the onions and stir well. Then pour the beer into the pan, stirring well to loosen all the caking from the bottom.

Place the meat back in the pan with the onions and beer. Add the bay leaves, thyme and brown sugar. Coat the bread slices on one side with mustard and place them mustard side down on the stew meat.

Bring to a boil, lower the heat and simmer gently with the lid on the pan for 2 to 2.5 hours, or until the meat is buttery tender.

Stir in the stew, removing the bay leaves and thyme sprigs. If necessary, add some more apple cider vinegar for a touch of acidity.

Season with additional salt and pepper.

Serve the stew with fries, mashed potatoes or a piece of crusty bread.