http://www.brandnewvegan.com/recipes/black-bean-tostadas

1 cup cashews, unroasted, unsalted, raw
¼ cup hot water
2 tsp Apple Cider Vinegar
2 tsp Lemon Juice
½ tsp Salt
½ tsp Red Wine Vinegar
4 Green Chiles, whole, peeled

1 cup cashews, unroasted, unsalted, raw 2 tsp Apple Cider Vinegar 2 tsp Lemon Juice 4 Green Chiles, whole, peeled

Cover 1 cup of raw, unroasted cashews with hot water and let them soak for 30 minutes. Or soak overnight

Once they are nice and soft, drain the water and add them to a blender. Add 1/4 cup of hot water, the vinegar, lemon juice, garlic, salt, and green chiles and blend on high until smooth.
Stop every now and then and scrape down the sides.
The longer you soak your cashews, the smoother the sauce will be.
I used about 4 whole canned green chiles, so they are naturally pretty mild. If you have access to fresh Hatch Green Chile, then I would just use one at a time until you get the taste (and heat) you like.
Using canned chiles my sauce was not hot at all, but still had that wonderful green chile flavor.
Awesome on the Cauliflower Tacos