Spent mushroom substrate is really good in compost (particularly vermicompost). I worked on a small scale gourmet mushroom farm earlier this year, it's easy enough to sell the spent medium to gardening businesses when you're done growing mushrooms.
I think all coffee grounds make decent fertiliser too, as well as decent compost, even without mushroom.
coffee grounds alter the pH of whatever you use them in so you can't just blindly use it as fertilizer. Just an FYI before people start throwing their coffee grounds everywhere. It's worth researching whatever your growing to see what pH it prefers and then treat accordingly.
They also just get moldy in "sterile-ish" environments like pots since there's nothing there to break them down. Works ok in the yard though.
Gourmet mushroom farm? Can you tell more?
It was at What The Fungus - you can check them out on youtube