Slice the plantains 3/4 inch thick on the diagonal. Heat 1 1/2 tablespoons each butter and vegetable oil in a nonstick skillet over medium-high heat. Working in batches, cook the plantains until golden, about 5 minutes per side. Drain on paper towels, reserving the skillet.
Put the flour in a shallow dish. Whisk the milk, egg, allspice and cayenne in another dish. Lightly crush the fried onions into a third dish. Dredge the plantains in the flour, dip in the milk mixture, then press into the fried onions, turning to coat.
Heat the remaining 1 1/2 tablespoons each butter and vegetable oil in the reserved skillet over medium heat. Working in batches, fry the plantains until golden and crisp, about 1 minute per side. Drain on paper towels. Serve with the hot sauce.