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Interesting find, I had never seen a white pluteus until today. Fruiting off old conifer logs, distinctive pink tinged gills. They smelled slightly cucumbery from a distance.

Cap: White, umbonate, texture smooth, pileipellis looks slightly fibrous. Center of the cap has darkened, brown fibres, radiating outwards. Cap 8.3cm in diameter.
Taste: Pleasant, a little nutty, turning sour, slightly acidic.
Gills: Closely spaced, salmon pink, younger specimens’ [gills] were white. Free, completely detached from stipe.
Stipe: White, bare. Fibrous, snaps similarly to celery.
Annulus: Not present.
Volva: Not present.
Spore print: Dull salmon-orange
Spore morphology: Thick walled, slightly angular.
Cheilocystidia: Present and abundant.
Pleurocystidia: Present, with apical prongs.
Caulocystidia: While observing shavings of the stem, I saw some meduloid-like structures. Not sure if they could have just been hyphae (Check image 12)

Images 1-5: Specimens in situ.
Images 6,7: Spore print.
Image 8: Spores, 2000x.
Image 9: Meduloid cheilocystidia on gill edge, 2000x.
Image 10: Drawing.
Image 11: Pleurocystidia and cheilocystidia together, 2000x.
Image 12: Possible caulocystidium on stipe, not quite sure, 500x.
Image 13: Cells on pileipellis, 500x.
Image 14: Pleurocystidia with apical prongs, 2000x.
Image 15: Cap from above.

P. petasatus arrived at via dichotomous key, generously provided by Michael Kuo on mushroomexpert:

https://www.mushroomexpert.com/pluteus.html

Observation #255