Preheat the oven to 350°. Cut the steaks so they fit side-by-side in a large roasting pan. Place the meat in the pan.
In a food processor, pulse the peppercorns, allspice, cloves, oregano, garlic powder, red pepper flakes, bay leaves, onion powder, mustard powder, mustard seeds, coriander seeds, cinnamon sticks and ginger until finely ground. Add the vinegar, Worcestershire, salt and water, and pulse to combine. Pour the spiced liquid over the meat, cover with aluminum foil and bake for 2½ hours. Remove from the oven and let cool in the braising liquid for 1 hour. Carefully remove from the liquid and pat dry.
In a large skillet, heat the vegetable oil over high heat until it just starts to smoke. In batches, carefully place the skirt steak in the pan and sear for 1 minute, then flip and cook for 1 minute longer. Transfer the steak to a cutting board. Add the red onions and lime juice to the skillet and sauté over high heat for about 2 minutes, or until the edges start to brown.
Transfer the steak to a platter, top with the onions and serve immediately.