Preheat the oven to 350 degrees F. Cut a lengthwise slit in each plantain and set on a baking sheet. Bake until the flesh is thoroughly soft and oozing through the slit, 40-50 minutes. Set aside to cool.
Meanwhile, make the stuffing: Peel, core and finely chop 3 of the quinces. Peel and core the remaining 2 quinces and cut each into 8 wedges. Drop the wedges into a bowl of cold water to cover and squeeze in the lemon juice.
Melt the butter in a small saucepan over moderate heat. Add the brown sugar and cook, stirring, until the sugar has melted. Add the finely chopped quinces, ginger, orange zest, cloves and cinnamon and stir to combine. Add 1 cup of water and continue cooking until most of the liquid is gone, about 15 minutes. Add the orange juice and rum and cook until almost all of the liquid is gone, about 5 minutes. Drain the quince wedges and add to the pan. Reduce the heat, cover and cook until the wedges are just soft, about 8 minutes. Remove the pan from the heat, stir in the walnuts and let cool. (The stuffing can be make a day in advance and refrigerated).
Make the dough in a food processor with a paddle attachment: Peel, trim and discard any tough ends from the plantains. Combine the plantains and salt and pulse until a smooth puree is formed, or mix until just blended. Wrap in plastic and refrigerate about 2 hours.
To assemble: Roll 2 tablespoons of the dough between your palms to form a ball. Line the bottom of a tortilla press with a small plastic bag and place the ball of dough in the center. Place another plastic bag over the dough and press to form a 3 1/2inch circle. Place a quince wedge on half of the dough circle and top it with a teaspoon of the sauce. Fold over to enclose the quince and press the edges to seal. Place the stuffed empanadas on a platter and chill at least 30 minutes.
Heat the vegetable oil to 375 degrees F. Gently drop the empanadas into the hot oil and fry until golden, about 6 minutes. With a slotted spoon, remove from the oil and drain on paper towels. Sprinkle with powdered sugar and serve hot.
c.1997, M.S. Milliken & S. Feniger, all rights reserved