What are hitachi mushrooms
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Merrill is a co-founder of Food52.
Any chance you mean shiitake?
I believe Merrill's hit it on the head. Shiitake mushrooms are common in Asian cuisine and can be found fresh at stores like Whole Foods (oh good, you already shop there!). They have broad, flat tops with rather spindly stems.
You can also find them dried in many stores. Those can be rehydrated with water or stock.
Overall, a VERY tasty mushroom - give 'em a shot and let us know what you think.