Ensure you use ripe plantains (yellow with black spots, nearly black). Unripe, green plantains would result in a bitter and overly starchy taste unsuitable for Pastelón.
The plantain puree can be made from boiled, mashed plantains. Alternatively, you can slice the plantains and fry them instead of boiling for a richer flavor, but this is a greasier version.
Peel the plantains, cut them into large chunks, and place them in a large pot with salted water. Bring to a boil.
Boil for about 15-20 minutes until the plantains are tender (like potatoes). Drain the water (you can save 1/4 cup for the puree if it seems too dry).
Mash the cooked plantains into a puree (like mashed potatoes). Stir in the melted butter, salt, and beaten egg. Mix well until the mixture is smooth and pliable. Set aside.
Heat the olive oil in a skillet. Add the onion, green bell pepper, and garlic. Sauté for 5 minutes until softened.
Add the ground beef and cook until browned, breaking it up with a spoon. Drain off any excess fat.
Stir in the tomato paste, water/broth, oregano, salt, and pepper. Reduce the heat and simmer for 10-15 minutes until the liquid has almost completely evaporated. The filling must be moist but not runny. Remove from heat.
Preheat the oven to 175 °C (347 °F). Grease a baking dish (approximately 20x30 cm).
Form the first layer: Spread half of the plantain puree evenly over the bottom of the dish.
Add the meat layer: Spread all of the Picadillo beef filling evenly on top.
Add the cheese: Sprinkle with half of the shredded cheese (Mozzarella, Cheddar).
Finish with plantain and cheese: Carefully spread the remaining half of the plantain puree on top. Sprinkle with the remaining cheese.
Bake in the preheated oven for 20-30 minutes, or until the cheese is golden brown and bubbly.
Remove the Pastelón from the oven and let it rest for 10 minutes so it can be sliced more easily and hold its shape. Serve hot as a main course with a simple green salad.