Season the beef chunks with salt and pepper and dust with flour.

In a large Dutch oven or heavy pot, melt 2 tablespoons of butter over medium-high heat. Add the beef in batches, browning on all sides. Remove the beef and set aside.

In the same pot, add the remaining butter and the onions. Cook over medium heat until the onions are caramelized, about 10 minutes. Add the garlic and cook for another minute.

Return the beef to the pot. Add the beer, brown sugar, apple cider vinegar, bay leaves, and thyme. Stir to combine.

Spread mustard on the bread slices and place them mustard-side down on top of the stew. This will thicken the stew as it cooks.

Bring to a simmer, then cover and reduce the heat to low. Cook for 1-2 hours, or until the beef is tender.

Remove the bay leaves, thyme, and bread. Stir the stew to make sure the bread is fully dissolved. Adjust seasoning with salt and pepper.