Have an old Corian serving tray that’s about 14” x 19” by 1/2” thick. It used to be flat but now it’s got a concave bow in both the length and width directions. Anyone know how to take a bow out of a piece of Corian? Heat? Heavy weights and time? Something else?

Plane it down.

Think I'd put it in the oven on fairly low heat while its laying on something that's flat and non-metallic (maybe a butcher block cutting board). Try lower heat and longer duration to see if the warp or twist will start to come out of it. You can always increase the heat if necessary. Given the twist and warp in it, and it being only 1/2" think, I don't think you're going to be able to successfully plane it down.

Heat and weight on top of it...……..don't overdue it on the heat......try maybe 140F in the oven.....

140 isn’t hot enough.

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