Pat beef dry and sprinkle with pepper and 1/2 teaspoon salt.
Heat oil and butter in a wide 6- to 8-quart heavy pot over high heat until hot but not smoking, then brown beef in 2 to 3 batches (without crowding), turning over once, until dark brown, about 5 minutes.
Transfer to a plate.
Add onions and remaining 1/4 teaspoon salt to pot and cook over high heat, stirring occasionally and scraping up brown bits from bottom of pot, until pale golden, about 5 minutes. Reduce heat to moderately low and cover pot, then cook, stirring occasionally, until onions are golden, about 10 minutes more.
Add beef along with any juices on plate, bay leaves, and beer and bring to a simmer.
Meanwhile, spread 1 tablespoon mustard on each side of bread slices, then place bread on top of stew. Simmer stew, covered, until beef is very tender, about 2 hours.
Just before serving, stir bread into stew, breaking it up with a spoon, until it is incorporated and stew is thickened. Season with salt.
Cooks' note:Stew can be made 1 day ahead and cooled completely, uncovered, then chilled, covered. Reheat over moderate heat.