Before your next batch of mushrooms hits the pan, give them a short sunbath. Fresh button mushrooms from the store usually carry less than 1 microgram of vitamin D per 100 grams. Their cells are packed with ergosterol, which plays the role cholesterol does in animals, and ultraviolet light converts it into vitamin D2, much like sunlight makes vitamin D in your skin. In a German test at midday in midsummer, sliced mushrooms reached 17.5 micrograms per 100 grams after 15 minutes outdoors and 32.5 after an hour. Slicing matters because it exposes more surface to the light, so slices beat whole caps given the same sun.
The honest caveat: results swing with season, latitude, time of day and cloud cover, so a gray spring afternoon won’t match those numbers. The good news is that the vitamin holds up afterward. A 2018 review in the journal Nutrients found sun-treated mushrooms usually stay above 10 micrograms per 100 grams through their best-before date, and pan-frying without oil for five minutes kept about 88 percent. Fifteen minutes on a sunny patio table, and your dinner borrows a trick from your own skin.