Stropharia rugoso-annulata Farl. ex Murrill
An introduced species found growing on wood chip and lawns where there is much organic content. Cap diam: up to 120 mm. Initially the caps are a red-brown colour, fading to a fawn as they age. The gills start as a mauve-grey but become inky black on maturity
These are edible - just slice them up and pan-fry them. Boil off the excess liquid. Yummm.