TheFork users who ate in Brussels have said really good things about Volle Gas, L'Ecailler du palais royal and THE FIFTY ONE - Tapas, Sharing Plates, Bistro. Maybe you should give them a try!
Some of the more popular Seafood restaurants that offer vegetarian options on their menu in Brussels according to TheFork users include:
Volle Gas, with a 9 rating
THE FIFTY ONE - Tapas, Sharing Plates, Bistro, with a 9 rating
Antartique, with a 9.1 rating