Score the peels of the yellow and green plantains lengthwise and remove and discard the peels. Slice 1/8 to ¼ inch thick lengthwise, keeping the yellow and green separate.
Lay the green plantain slices flat in the bottom of a large pot and fill it with water. Make a bed of paper towels for draining. Bring the water to a boil over high heat, salt it to taste like the ocean, and boil the plantains until tender, about 30 minutes. Drain them and put them on paper towels to dry.
In a large, straight-sided sauté pan, heat 2 inches of oil over medium-high heat until it reaches 350°F. On the counter, make a bed of paper towels. Slide the yellow plantains into the oil and fry until they are golden brown but still soft inside, 8 to 10 minutes. Transfer to paper towels to drain.
Heat the olive oil in large skillet (preferably cast iron) over medium heat. Add the onion, season with ¼ teaspoon of the pepper, and cook, stirring often, until tender and translucent, about 10 minutes. Add the chile verde, beef, chorizo, saffron, achiote, oregano, and cumin, and cook until the meat is cooked through, breaking up the meat with a wooden spoon, about 5 minutes. Stir in the hard chorizo and olives. Turn off the heat and strain the fat from the meat, spooning out the grease with a large spoon. Set aside while you prepare the other components of the casserole.
Grease the bottom and sides of a large baking dish with butter. Lay the plantain slices in the dish in a single layer as you would noodles for lasagna, alternating between green and yellow. Spread 1 cup of the meat mixture over the plantains. Lay down another layer of the plantains and another layer of meat until you have built three layers and used all of the plantains and meat.
Whisk the egg yolks, cream, and remaining 1/2 teaspoon pepper together in a small bowl or glass measuring cup and pour evenly over the casserole. Scatter the cheese over the top. Bake the pastelón for 20 to 25 minutes, until the cheese bubbles and browns and the egg is set. Set the pastelón aside to cool for 5 to 10 minutes before serving. Serve hot, cut into squares, with the béchamel
on the side.
For the béchamel: Melt the butter in a medium saucepan over low heat but be sure it doesn’t brown. Cook the shallots with the salt, stirring often so the shallots don’t brown, until they are tender and translucent, 4 to 5 minutes. Stir in the flour, whisking until no lumps remain. Cook, stirring constantly, until the flour is very light golden, about 3 minutes. Gradually add 1 cup of the milk, whisking until incorporated. Pour in the remaining milk and a good pinch of salt, and bring the milk to a boil. Reduce the heat and simmer, stirring with a rubber spatula so the sauce doesn’t scorch on the bottom of the pan, until it is thick enough to coat the back of the spatula.
Stir in the corn and cook for about 2 minutes to warm it through. Add more salt to taste.