1Cook frozen broccoli as directed on bag. In 3-quart saucepan, melt cheese over medium-low heat, stirring frequently. Stir in broccoli, beer, pimientos and mustard. Cook until hot, stirring occasionally.
2Pour cheese mixture into ceramic fondue pot over low heat or 1 1/2-quart slow cooker on Low heat setting.
3To serve, top each bread slice with sliced roast beef; spoon cheese mixture over beef.