I would write 2 letters of complaint - one to the manafacturers and one to the supermarket. Keep it brief and say when and it was purchased, the sell by date, and how horrified and disappointed your were to make this discovery, and that you are a loyal customer etc etc etc. You should get something from both parties. You could also send the offending goods to Old El Paso, but I wouldn't normally bother. Good luck
don't send your evidence, there is always a chance of them ignoring you and then you will have no proof after sending it to them. treaten to go to your local news papers if they ignore you. but do what scoobysoo said.