BRITTLEGILL MUSHROOMS : genus Russula
There are around 190 species of Brittlegill mushroom in the UK so we can offer no more specific identification. If you want it, you will need to refer to an expert. As the Mushtoom Table website tells us : They are easily recognised as a genus but not easy to identify to species. Brittlegills have a distinctive clean shape with a central stem in the centre of a mostly circular cap and gills which are never decurrent (running down the stem a short distance). Only about 20 of 190 UK species can be identified from field characters. The others need at least a spore print and some chemical test.
Brittlegills are mycorrhizal. This means they are associated with particular trees. Always take note of the species of near by trees as his will often help with identification. Some Brittlegills only grow with one type of tree, others are more generalist.
An example growing in a lawn amongst various trees including a conifer.
And its underside, the gills typically fragmenting and the edges appearing to have been nibbled.
MUSHROOM TABLE website : https://www.mushroomtable.com/mushrooms-menu/wild-mushrooms/brittlegills/