I had one that was warped pretty bad. It eventually righted itself by laying it 'flat' (I don't remember if the convex side was up, but I think it might have been?) over a pot after soaking/washing it thoroughly, this allowed it to dry more evenly with air flowing underneath and it knew what to do.
- I did not use direct heat, but it was there for a couple days next to the other 3 burners
- I did not use any weights
- I did not steam
- the pot was completely empty the whole time, with nothing in it but cycling air.
- The board did not cover the pot completely, allowing air to circulate around it.
- I repeated this process a couple times to get it ~96.2% back to it's original 'flatness'
If you have electric burners (coils) (probably easier/safer than gas for this purpose) then there are many less than safe, supervision required, ways to accelerate the process mine went through. Of course you could also rig something in the oven. Note that accelerating the process could also cause it to warp more. Clearly this phenomenon was not well documented, but I wanted to let people know it's possible.