In this robust soup, the sharpness of the lime juice and tomatillos is a wonderful counterpoint to the trio of herbs. The original at Brisas del Mar used the head of a big robalo to make the all-essential stock. This adds a lot of gelatin and flavor. Use a fish head if you can get one, but never from an oily variety like salmon or bluefish. You could substitute fresh tarragon for the hoja santa.