After a week of working up the courage to eat shellfish, plus a recommendation by the receptionist at the Breughel hostel, I finally had moules and frites (mussels and french fries). Mussels are out of season, so a local wouldn't be eating them. But I'm a tourist and won't be here in-season.
The Brit father of the family at the next table said the mussels were very good, but the frites we below par, so I'm inclined to think the mussels have room for improvement as well. The same waiter I had showed the Brit father how to use a used mussel shell to hold on to a new mussel to eat, which I assumed was a better, local way than the spoon I was trying to use. Overall, I'm satisfied with the experience because Chez Leon at least feels authentic compared to the touristy places that tried to pull me in as I walked here. The napkins say since 1893, so I suppose that's something. Maybe I'll find out at some point in the future how good of an experience this was today.