Similiar to Ethel's reply (if you haven't any large trays with deep sides) place in bath with very hot water and dishwasher powder, leave to soak and it will come away very easily.
I don't have a dishwasher, but several years ago someone advised me that it is great for various jobs like the one mentioned. I've used it for tea/coffee stains in cups and mugs (drink both black, so it soon discolours them), and any pans (not aluminium) that need a clean. It lasts a long time because you do not need to use that much.