Sumac is a spice that is popular in the Middle East. One of the more common uses of sumac is in the spice blend, zaatar. Sumac is both sweet and sour, much like the flavor of fresh lemon juice. I love to use it in recipes that could use an acidic punch, but in which I don't want to add extra liquid. As the spice increases in popularity, you can sometimes find it in the spice section of major grocery stores. Otherwise, you can order sumac on Amazon. You can substitute fresh lemon juice for sumac in this recipe. Simply add a couple tablespoons of lemon juice to the mushroom ragù when you add the mushroom soaking liquid.