Light a charcoal fire and let it burn until all the coals are medium- hot and covered with gray ash. Nestle the onions directly in the coals and let them roast until charred on the outside and soft within, about 20 minutes.
While the onions are roasting, pound the meat with a flat mallet to about 1/8-inch thick. Drizzle with 1/4 cup of the sour orange (or 3 tablespoons lime) juice, cover and set aside. Cool the onions until handleable, peel the charred outer layers off, then cut what remains into 1/2-inch squares. Toss with 1/4 cup of the sour orange (or about 3 tablespoons lime) juice and season with salt and pepper. Toss the cabbage with the cilantro and the remaining 1/4 cup of the sour orange (or 3 tablespoons lime) juice; season with salt and pepper. Spread onto a large serving platter about 14-inches or so in diameter. Decorate the perimeter with alternating slices of tomato and avocado, and sprinkle them with salt and pepper.
Stoke your fire with new charcoal and let it get really hot. Sprinkle both sides of the meat generously with salt and pepper. Working with a couple of pieces at a time, grill the pork: let it sear about 2 minutes on one side, then flip it over and sear on the other. (The total cooking time for 1/8-inch pork steaks over a very hot fire will be no more than 3 or 4 minutes.) Lay the meat slightly overlapping down the center of the platter and strew the onions over the meat. Decorate with a few cilantro sprigs and serve without delay, accompanied by salsa and baskets of hot tortillas.
Variations and Improvisations
This simple dish could easily be made with butterflied, pounded, boneless, skinless chicken breasts or with skirt steak. If you like spicy food and want more flavor in the marinade, smear a little habanero salsa over the meat in place of the sour orange.