In a medium dutch oven, bring the stock to a boil over high heat. Add the chard stalks and simmer for 4 minutes, or until they start becoming tender. Add the chard leaves, zucchini, and salt. Replace lid and simmer for another 4 minutes or so. When the vegetables are beginning to soften, but are not completely cooked through, add the cilantro and cook for an additional minute (you just want the herbs to wilt slightly).

Remove greens from pot, and set cooking liquids aside for another use. Blend the greens in a food processor or blender, adding some of the liquids as necessary to obtain your desired texture.

Add the lemon, cayenne, and taste for seasoning.