Cut the pork belly and pork neck into fist-sized chunks. Place all pieces into a large bowl.
Using a pestle and mortar, crush the white peppercorns, black peppercorns, oregano, and coriander seeds into a fine mixture.
In a food processor, combine the orange juice, olive oil, garlic, rosemary, sea salt, and the crushed spices. Blend until smooth.
Pour the marinade over the pork and massage it into the meat. Cover, refrigerate, and marinate for 2-12 hours for best results. Before skewering, bring the meat back to room temperature.
At least half an hour before cooking, prepare the charcoal. Ensure it's hot and ready when you place the spit on the souvla.
Skewer the pork, starting with a prong or red onion/ white onion to hold the meat in place. Alternate pieces of pork belly and pork neck, start and finish with a fatty piece. Close off the spit with another onion or prong.
Cook the pork on low-medium heat for about 1.5 - 2 hours. Then, increase the heat to medium-high by adding more coals or lowering the spit closer to the fire and or scratching the charcoal. Cook until the pork reaches an internal temperature of at least 70°C (160°F) or around 80°C (175°F) for a more well-done finish.
Remove the spit from the grill, cover the pork with foil, and let it rest for 10 minutes. Carve and enjoy!