Method:
Put orange juice, green onions, vinegar, tamari, lime juice, peppers, allspice, cinnamon, nutmeg and garlic into a wide, shallow dish and mix well to combine. Add pork and turn to coat well. Cover and chill 4 hours or overnight.
Oil grill grates and preheat grill to medium heat. Drain pork and discard marinade. Grill pork, turning occasionally, until just charred in parts on the outside and cooked through, about 15 minutes; transfer to a platter when done. Brush pineapple and plantains with a bit of oil then grill, flipping once, until deep golden brown and hot throughout, 5 to 8 minutes; transfer to a platter as done.
Cut pork crosswise into medallions then transfer to plates and serve with pineapple and plantains on the side.