Research has shown that exposing sliced white button mushrooms to direct sunlight before cooking dramatically increases their vitamin D2 content because UV-B light converts ergosterol in mushrooms into vitamin D2. Slicing the mushrooms first increases the exposed surface area, making the process more effective.
One well-known study found that just 15 minutes of midday sun significantly increased vitamin D2 to levels comparable to fatty fish (about 704 IU per 100 g of fresh mushrooms&). Other studies have reported even larger increases depending on sunlight intensity and exposure conditions.