On a plate rub 1/2 tsp of oil and a pinch of salt all over the corn and the cut sides of the avocado. In a small bowl, combine the garlic, oregano, cayenne, lemon zest, 1/4 tsp salt and remaining oil. Place the fish on a plate, pour half of the marinade mixture over the fish and work to coat all sides. Wash your hands before the next step.