Notre-Dame Cathedral in Tournai by Joel on the Road
My wife and I visited here in August 2017. The cathedral is currently undergoing heavy renovation, both inside and out. I would estimate something like half of the exterior is covered in scaffolding, and less than a third of the interior is available for viewing.
However, you can still see the impressive towers and the portico at one end, as well as the exposed remains of earlier churches inside.
With that in mind, Tournai is actually a beautiful little town for a visit, particularly on a sunny summer afternoon like we had. A great lunch on the square, a climb of the belfry (part of the separate Belfries of Belgium and France listing) from where you get a fantastic view of the cathedral, then a wander around the cathedral.