By THÉRÈSE MAGOLIS     The upscale El Lago restaurant in Chapultepec Park has created a special Christmas holiday tasting menu that combines seasonal delights with traditional Mexican delicacies. The new seven-course includes a tuna tart in mango and balsamic vinegar reduction, a turkey hash with caramelized sesame seeds, a buttered cream of pumpkin chowder with morita chillies and hazelnut mousse, a red