Wash and peel the green plantains, be careful as green plantains tend to stain clothes and cutting boards. It works best to do this under running cold water.
Cut three of the plantains in half. Keep the remaining peeled plantain in a bowl of cold water.
Bring water to boil in a large saucepan and add the 3 plantains. Cook until tender, about 30 minutes. Exact time will vary based on the greenness of the plantains.
Remove from heat and let the cooked plantains sit in the water they cooked in until they are cool enough to handle (but still warm), if you remove them from the warm water they will get too hard.
In the meantime, grate the remaining raw plantain using a very fine grater.
Place the cooked plantains in a food processor and pulse until you have a thick coarse mix. Add the egg and butter or oil, and pulse until the ingredients are well mixed and the dough is a little smoother. If you don’t have a food processor, you can mash or grate the cooked plantains and then mix them with the egg (lightly whisked) and the butter.
If using the food processor, remove the plantain mix and place it into a large bowl. Rub your hands with a bit of butter or oil, and mix in the finely grated raw plantain and ½ to 1 teaspoon of salt (adjust based on how large the plantains are and your level of tolerance for salt). Use your hands to work the dough until you have a smooth mix. If the dough is very dry or too sticky, you can add a bit of the cooking water from the plantains – you will likely need to this if you didn’t use an egg.
Form the dough into a ball and let rest in a bowl at room temperature for about 15-30 minutes. You can also save it to use later and store in the refrigerator for 24 hours, but when you remove it from the refrigerator, let it rest for at least 30 minutes at room temperature.
To roll out/make the green plantain empanada discs:
As mentioned above you can make the empanada discs by either using a rolling pin to flatten the the dough into a thin sheet. The dough can be difficult to handle, use oil on your hands and on rolling pin to keep the dough from sticking. Use a round pastry cutter mold (or round bowl or cup) to cut out round disc shapes. The size of the empanadas is up to you, I usually make them medium or small sized if I’m serving them as appetizers or sides, or large if I’m serving them as a main meal.
The other option for making the empanada discs, is to shape the dough into small balls – golf size for small ones and tennis size for larger ones. Then flatten these balls of plantain dough using a tortilla press or something flat, like a cutting board. You can also use a rolling pin to roll out each individual ball into a round disc – don’t worry if they aren’t perfectly round. It helps to place the dough balls in between pieces of wax paper (lightly greased) to keep it from sticking to the board, tortilla press, or rolling pin.
For the filling and assembly:
Mix the cheese and the onions together.
Place a spoonful of the cheese and onion filling in the middle on the empanada disc, fold and seal the empanada. You can also do this using a piece of wax paper. To seal it correctly press the edges gently with your fingers. You can also use a fork to help seal the edges.
Let the empanadas sit in the refrigerator for about an hour, this will help them seal better. It is possible to fry the empanadas immediately, but they are more likely to open or leak while frying.
Heat the oil in a large frying pan over medium high heat. As mentioned before, you can deep fry them or you can use just a few tablespoons.
Add the empanadas, do not overcrowd the pan, and fry them until they are golden and crispy. If using a small amount of oil you will need to gently flip them over so they cook on each side, about 3-4 minutes per side.
Drain the oil from the empanadas by placing them on plate lined with paper towels.