It's one of the brown species of
Melanoleuca. There's a good chance it's
Melanoleuca polioleuca,
Common Cavalier (which is included in older books under the incorrect names
M. vulgaris or
M. melaleuca which is actually a much less common species). To be sure which species you have you need to do some microscopy, looking in particular for the presence or absence of cells on the gill called cystidia.
But a good tip for the field is to cut the mushroom in two. If you have dark brown flesh in the stem it is likely to be
M. polioleuca because
M. melaleuca has pale flesh in the stem.
Ken