Cilantro roots are an important part of Thai cooking. They are used to add a rich deep earthy floral essence to marinades, sauces and to flavor ground meat. Hard to find at conventional markets, I have had amazing luck with the farmers market. The wonderful thing is that they can be frozen so you can have them all year long. Be sure to buy plenty if you find them. Cut and use the cilantro leaves and stems par usual. Wash the roots well and place in a ziplock bag, the air squeezed out and well sealed. Freeze. Thaw them a bit before using. Most recipes require them to be blended or finely chopped and the freezing actually helps to break them down a bit.