Add celery, carrots, and onion; cook, stirring occasionally, until onion is soft (about 10 min.).
Stir in flour, mustard, and thyme and cook for one minute.
Gradually add stock.
Bring to a boil over medium-high heat, stirring often; reduce heat, cover, and simmer, stirring occasionally, until vegetables are tender (12-15 minutes).
Stir in cheddar and parmesan cheeses; when melted, add beer.