In a pot, place the flank steak, water, salt and pepper. Cook over medium heat for about 1 hour and 15 minutes or until the meat is cooked.
Measure 1 cup of the liquid used to cook the meat and set aside. Remove the beef from the pot, let it cool and shred with a fork or your hands.
In a saucepan, place the olive oil, onions, bell pepper, tomato, and garlic. Cook for about 5 minutes. Add the tomato paste, cumin, shredded beef, and the 1 cup of reserved beef liquid. Cook for 6 to 8 minutes, stirring occasionally.
To assemble the empanadas: Place a spoonful of the ropa vieja and diced plantains on the center of the empanada discs.
Brush the edges of the empanada discs with the beaten egg. This will help them seal better and keep the filling from leaking while baking.
Fold the empanada discs and seal the edges. Use a fork to help seal the empanadas or use your fingers to twist the edges. Lightly brush the top of the empanadas with the egg, which will give them a golden color when they bake.
Chill the empanadas for at least 30 minutes. Preheat the oven to 400 F and bake for 15-20 minutes or until golden.
To make the avocado sauce: Place all the ingredients in a food processor and process until smooth.
Serve empanadas warm with a side of sauce.