Preheat oven to 400 degrees. Cut down the center of the pork loin, lengthwise, making a nice vertical slit with cutting the pork loin in half. Place the butter, chopped onion and basil into a skillet and saute over low heat. Add the spinach, stirring together until heated, then add the Parmesan cheese to this mixture and stir together just until the cheese melts. Stuff the mixture into the slit in the pork loin, filling the entire length of meat. Roast in the oven for 40 minutes in a covered roasting pan. Serve hot or cold. Note: if you don't have fresh spinach, you can use a box of frozen chopped spinach, thawed. Fresh Parmesan is preferred but not absolute. If you don't have a covered roasting pan, you can tie the roast together and back uncovered.