Cap 3 to 10cm, domed and remaining so a long time before expanding fully, white sometimes creamy yellow. Stem with a ring that quickly disappears. Gills deep pink even in unopened 'buttons', finally darkening to brown; spore print dark brown. 

There are several other similar mushrooms (some of which can make you very ill)

Photo from top and side, and underneath

Meadows and grasslands, particularly where horses graze.

Autumn

Formerly very common but now less so following the decline in the use of working horses on farms.

Fairly common in Leicestershire and Rutland but declining.

https://www.first-nature.com/fungi/agaricus-campestris.php