I freeze cook mushrooms on a regular basis since I do a lot of bulk/cook/freeze stuff and then microwave it back up. Never had a problem with mushrooms. Most often they are sort of mixed in with egg. Posh people might call it an omelette but since I do loads in a large deep frying pan it looks a lot like a mixture of eggs and mushrooms. I just slice and freeze it, add a couple of high meat content sausages all into a Chinese chippy container (bought new from Ebay) and, hey presto, breakfast for a week.