Try a heavy guage enamel baking tray. I've had mine for nearly 15 years now and it stays flat as a pancake. I think I may have seen them in Lakeland. I also have a slightly smaller baking sheet which is non-stick which I also got in Lakeland (I think) and it also stays perfectly flat.
Will be watching this.. All my pans bend up. Ithought it was the way I use it, because the up side is always dry. I have wilton and they do it..and I thought they were pretty good.
I have this problem with a baking sheet and a low-sided baking pan (both different brands). Each time I'm baking something at a temperature of 200C or over, the corners bend upwards while in the oven so one side of the food being baked starts browning/cooking faster than the other side.
Any suggestions for brands of baking sheets that don't suffer from this issue? Probably best to get something used in commercial kitchens so if anyone knows of a restaurant equipment supply store, that would be great!
Thanks.