Although I not that fond of Brussels (and Belgium generally), the one place I do remember is Noordzee. I do fondly recall standing around the plaza during a very busy lunchtime as the rush of soup, fried and grilled seafood was being thrown around all over the place. It was a wonderful experience and one I wanted to try again (http://eatlikeushi.posthaven.com/de-noordzee-la-mer-du-nord-brussels-02-2012). The prices have gone up a bit, but what can you do?
They close at 6pm which means it's difficult to go for dinner. My first visit this trip was at 5pm when there was much less variety available. Nonetheless the fish soup €6 (chunks of fish in a tomato based broth with melted cheese and toasted baguette with garlic rouille similar to bouillabaisse - excellent) and the scampi a la plancha (cooked simply to a nice juicy firmness with olive oil, salt, garlic, parlsey and lemon) €8 reminded me of why I enjoyed it so much.
I went again the next morning at 10.30am where there was nobody around and a full menu to pick from. Fried kitterling €5 (soon they changed the price to €6, fried small pieces of white fish in batter), razorclams €9 (served undercooked but I didn't mind, with the same excellent seasonings as the prawns the day before) and the de-shelled escargots €6 (served in a chilli-infused broth) was all great.
There is lots of fresh and toasted bread to pad out the meal and fill you up. The flavours remain fantastic.