Beech kitchen worktop - bowed

Bring it indoors I guess. If it was originally flat in those conditions, it should be again

Yep. Get it into low humidity and wait.

Then screw it down HARD on assembly.

Timber has an awesome power when it wants to distort.

Had a job recently where the customer had built wardrobe doors by glueing T&G timber horisontally across a sheet of 6 x 2 Contiboard. He had glued every strip instead of glueing only the top & bottom strips.

Three months later they had bowed to the point where the ends where a good

Eventually sorted it by using trestles & a heavy suitcase to take out most of the bow, then fixing a 2 x 1 batten on edge - glued & screwed.

hammering wedges into rock cracks and dousing with water is a well known technique for quarrying.

My brother had the same problem. The carpender cut slots down the back to make the worktops flexible enough to screw down. I don't know if he fitted bracing pieces or not, or just fixed to the units directly. He warned that this might cause the worktops to split, but there wasn't really any other option other than chucking them out anyway. Several years on, they're fine and you'd never know.