To cook the pork: In a large sauce pan, heat the oil over high heat, add the strips of pork loin and cook for about 10-12 minutes or until browned, stirring frequently. Keep aside on a warm plate.
In a small bowl mash the avocado and mix with lime juice and salt. Keep in the fridge until ready to assemble the arepas.
To make the arepas: In a medium bowl add the water, arepa flour, butter and salt. Mix with your hands, making circular movements while breaking the lumps that may form with your fingers, until you have a dough. Let rest for 5 minutes.
Preheat a non-stick skillet over medium heat. Form balls and flatten them gently until they’re about ½ inch thick discs.
Place the discs on the skillet and cook the arepas for about 3 minutes on each side or until lightly golden brown. Cover them with aluminum foil to keep them warm.
Split each arepa in half and fill them with pork, avocado mixture, cilantro and onion slices and serve.