Any ideas on how to remove stubborn food stains from microwave containers?

DH is a shocker for letting them sit at work, bringing them home days later, stained to the hilt .

I've tried the "can you please wash the container when you've finished" line but "he's too busy".....

They aren't tupperware containers (but funnily enough the one tupperware dish I do own doesn't have this problem).....I feel a shopping spree coming on .