The extreme flocculence of the cap margin and the smooth bulb suggest A. cinereoconia to for this mushroom. Also, the gills are still white after the cap margin is flaring up. In A. onusta, the gills turn yellow and start to decay very quickly and would probably be translucent and yellow by the time a specimen had expanded this much.

I think cinereoconia is a good possibility here.

Very best,

R