As you may have heard, scientists have been finding that most of us aren't getting enough vitamin D—previous nutritional recommendations were too low. We can get our Vitamin D either from meat or the sun, created by our skin when exposed. But if you're one of the many who have been cutting down on meat in your diet and covering up to protect yourself from skin-damaging UV, you've limited the major sources.
Luckily, there's a food that's neither vegetable nor meat that can help fill our requirement—it's mushrooms. It's amazing that fungus is biologically more similar to animals than plants. Case in point: it turns out that mushrooms create vitamin D from sunshine, too. Amazingly, you can put a basket of mushrooms in the sun for 5 minutes and increase the Vitamin D by 80 times! This transforms the humble button or cremini from a weenie to a powerhouse source of D—up to 400 IUs of D in a single cup! If you leave the mushrooms for an hour, or slice them to expose more of the surface to sunlight, they generate even more vitamin D. You can then store the little guys until you're ready to use them—the D will degrade about 25% in the first three days, then stabilize.
My source, Eugenia Bone, in her book Mycophilia, says we'll be seeing lots of fortified mushrooms on the market in the coming years, but you can do it yourself at home on a sunny window sill, and so easily!