It is surprisingly difficult to find a highly commendable place in Brussels to eat mussels. I remember the research from my first trip in 2009 that took me past all the restaurant touts in Rue de Bouchers, where the atmosphere is lively but the food is apparently substandard tourist traps. The two places in the area that are decent are Aux Armes de Bruxelles (which is expensive) and Chez Leon (which I tried in 2009 and felt the atmosphere was of a high turnover tourist venue also).
Hence I ventured further outwards and read about La Roue D'Or, which had moderate-to-good reviews rather than superlative (but again, I couldn't find any online).
The interior is quite impressive with ornate watercolour decoration. The moules marinieres (€24) was pretty good too, topped with lots of onion and celery. The fries were reasonable but could have been a bit crispier. Grain bread was supplied to help soak up the liquid. I probably didn't become a true mussels convert until Dubrovnik later in the year, so I'd have to come back here again to give a new opinion. Unfortunately the cost of food in Belgium isn't as low as Croatia, so it'll have to be a good Euro exchange time.