In a blender or a bowl using an immersion blender, mix all the marinade ingredients (orange juice, lime juice, garlic, oregano, cumin, olive oil, coriander, mint, salt, and pepper). Blend into a smooth sauce. Add the meat to a freezer bag and add two-thirds of the marinade sauce. Set the rest of the sauce aside. Place the meat in the fridge for 3 - 4 hours or overnight.
Place a rack in the middle of the oven and preheat it to 240 °C / 465 °F. Add the meat and half of the marinating sauce to a large cast-iron skillet or pot. Add warm water and halved peeled onion. Cover with a lid and place in the oven. Roast for 20 - 25 minutes at 240 °C / 465 °F. Lower the heat to 170 °C / 340 °F and roast for 2 hours and 30 minutes. When the meat is roasted, remove it from the oven and set aside for 20 minutes.
Using a fork, shred the meat into larger pieces. Add the saved (unused) sauce and five tablespoons of those delicious roasting juices. Stir to combine.
Cut the bread into six even sandwiches, approximately 15 - 18 cm / 6 to 7 inches long. Spread mustard on both cut sides of the bread. Arrange the sliced gherkins over one side of the bread. Add two slices of honey-glazed ham, cheese, and two generous tablespoons of roasted pork. Drizzle with roasting juices and seal the sandwich. Press together for the sandwich to flatten. Spread with melted butter on top. Place the sandwiches in the skillet, top with a baking paper, then weigh it down with a heavy skillet. Cook for 2- 3 minutes per side or until golden-brown and crispy outside with melted cheese inside.