This is a technique popular in parts of South and Central America. Banana leaves can often be found in the frozen food section of Asian supermarkets. Annatto is a natural red food colouring with a slightly nutty taste - if you can't find it, substitute turmeric. Use pork that is not too lean or the dish will be dry.
In a bowl, rub the pork with 1/2 teaspoon salt. Mix the lemon and lime juices together and sprinkle one tablespoon of the juice over the pork. Set aside.
In a small frypan, toast the peppercorns and cumin until browned. Grind the seeds in a mortar and pestle or spice grinder with the annatto or turmeric.
Add the garlic and remaining salt to the grinder and mash well. Add the oregano and remaining juice. Mix.
Toss the pork with the spice paste and the onion.
Cut the banana leaves into four equal sized pieces. Pass the leaves quickly over a hot stove burner until they become shiny. This makes them easier to use.
Lay the banana leaves in a baking dish so that their ends overlap the sides. Place the pork in the middle and tightly fold the leaves over to make a package. Cover with foil and refrigerate for 4 hours.
Place the baking dish in a cold oven, then heat to 200 degrees C and roast for 90 minutes. To serve, discard foil and banana leaves and serve with white rice or tortillas and salsa.