Tricholoma nudum, Rhodopaxillus nudus, or Lepista nuda, pied-bleu is a tasty, strong-flavoured autumn mushroom that is often abundant until the first frosts in broad leaf or conifer forests. The stem is usually lilac or light violet in colour rather than blue.
Raw ones can lead to indigestion, so be sure to cook them; they are good for stewing.
Image by Lukas from London, England