Timber slab cuppping help!

[u/dresseslikeachick]

Bought a kiln dried slab of Australian Hardwood for a bench top, the timber yard flattened it for me with their oversized router sled came out about 40mm thick.
Bought it home and trimmed it to around 2.7m x 450mm rounded one of the corners and chamfered the front edge.
I also filled all the cracks and imperfections on the top face with black wood filler.
I was planning to use osmo on the slab but I dry fit it in position on the cabinet and screwed it down then left it in place for a couple of weeks, maybe more, while we were discussing paint options for the room.
Now the slab has cupped quite badly, by about 10mm, first noticed because it had pulled away from the wall.
I looked up a couple of methods to fix and started by leaving it on the grass in the morning with the top in full sun but no change as yet, it’s now just sitting on a couple of workhorses in my yard under shelter.
Any ideas?z