Looking for a large hall for a social gathering

Loads.
Den Engel/parochiezaal, Tervuren; Le Bouche à Oreille, Etterbeek
http://www.ceremony.be/
https://www.onehousestand.com/
https://www.salles.be/
https://www.1001salles.be/salle-des-fetes-2-belgique-p.html
https://www.mariage.be/location-salle/salle-de-mariage-belgique-bruxelle...
https://www.chouxdebruxelles.be/fr/salles-devenement/

Are you looking for something cheap? (Kamp Kwaadraat Overijse)
Middle cost?
Or expensive? https://www.martinshotels.com/en/hotel/chateau-du-lac/meetings-more

Lots of options depending on the above

ceremony.be

how about something free ? Like for a club.....maybe 20-30 people.

Overijse and the Chateau du Lac are NOT anywhere on the STIB network
A free club for 30 people is NOT suitable for 250
You do NOT need to repost a link that has already been posted

Geez peeps - do you actually bother to read the damned question before you respond????

My company had its year-end party at Docks Bruxsel. I guesstimate there were around 200-250 people, and it is easily accessible by STIB trams and buses.

Ground floor of Kanal-Centre Pompidou/Street Food Market