Onion, cilantro, oil, lime juice, garlic and orange juice are mixed in a bowl. The tilapia is sprinkled with pepper and Kosher salt. Half the onion mixture is spread on the bottom of the baking dish. The fish is put on top of the onion mixture and the rest of the mixture is spread on the fish. This preparation is covered and refrigerated for 30 minutes. Then the fish is turned over and chilled for 30 more minutes. The milk, mayonnaise, and remaining lime juice is whisked in a bowl.
The grill grate is brushed with oil and the barbeque is prepared. The fish is grilled with the marinade for approximately 5 minutes on each side. The tortillas are grilled for 10 seconds on each side.
The tilapia is coarsely chopped and served on a platter. It is served with mayonnaise mixed with lime juice, remaining onions, tortillas cilantro, cabbage, avocados and wedges of lime.