some kind of Russula sp. In a triangular grassy area fenced off, east end of Delacorte Theater and the bird watching dock on Turtle pond, near the main sidewalk that circles the ballfields.
There's another one about to emerge, brown cap a few feet in front of it. Spotted this after the NY Mycological Walk led by Gary Lincoff, and on my way to see Gil Scott Heron at Summerstage. A great but very hot day!