Highly recommend:
Yves Mattagne's Sea Grill - 2* Michelin, so not cheap (think easily €500+ for two without exploring very far up the wine list), but some of the best cooking we have had since leaving Japan. We ate the multi-course 'Menu Incontournable' and had the matched wines, then went back on the Monday to eat lobster presse (made with one of five Christoffle lobster presses in the world. which was also excellent but isn't served at the weekend). Great seafood, exquisite presentation, interesting flavours - some from Asia others from Africa and the Middle East.
Avoid:
La Truffe Noir - 1* Michelin, probably the least impressive 1* meal I have eaten. Yes it involved lots of truffle, but the service was poor, wine pairings poor value / uninteresting, compounded by my other half finding a decent sized piece of gravel in one of the dishes which was then not handled well ("bof, it must have been in the truffle..."), then them abandoning us at the end of the meal while they chatted with some mates who had been in celebrating a birthday was the icing on the cake.
If you are looking for a slice of old school Brussels cooking (e.g. horse steak, moules, etc.) in more of a canteen style, then we found Le Pre Sale at Rue de Flandre 20 to be OK for what it was - very traditional, clearly a Brussels institution - but maybe not what I would call a 'decent dinner'.