Wash leeks in a bowl of cold water, agitating water, then lift out leeks and drain in a colander. Cook leeks, carrots, celery, garlic, and bay leaf in butter in a 4-quart heavy saucepan over moderate heat, stirring occasionally,
until vegetables begin to soften, about 5 minutes. Reduce heat to moderately low and sprinkle flour over vegetables, then cook, stirring occasionally, 3 minutes. Add milk, broth, and beer in a stream, whisking, then simmer, whisking occasionally, 5 minutes. Stir in Worcestershire sauce, mustard, salt, and pepper.
Add cheese by handfuls, stirring constantly, and cook until cheese is melted, 3 to 4 minutes (do not boil). Discard bay leaf.
Serve sprinkled with bacon._____________