BLEND water, chiles and onions in blender until smooth; strain. Discard solids; set strained liquid aside.
HEAT dressing in 6-qt. Dutch oven or large deep skillet on medium heat. Add chicken; cook in batches 5 min. or until browned in all sides, turning occasionally. Add corn, zucchini and epazote leaves. Pour strained chile liquid over ingredients in Dutch oven. Bring to boil; cover. Simmer on medium-low 20 min. or until chicken is done (165ºF) and vegetables are tender.
SERVE topped with sour cream.