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Chlorophyllum molybdites
Looks to me like a dry young specimen (I say "dry" because of how the gills are a mess near the edge of the cap and I say "young" because of the yellow gills that eventually turn gray or greenish.) It's growing on the ground near grass, has the brown-patches-on-white-cap thing going on, is sporting the uplifted and depressed cap shape, the brownish-reddish stalk is complete with a persistent ring, and the gills are free (not at all touching the stem). Cosmopolitan and poisonous -- not a good combo!