How to Make It
In a small bowl, cover the raisins with hot water and let stand for 10 minutes. Drain well.
In a large saucepan, cover the chicken breasts with water and bring to a boil. Reduce the heat to low and simmer until just cooked through, about 25 minutes. Transfer to a large plate and let cool slightly. Pull the meat from the bones and tear it into thin shreds.
Meanwhile, melt the butter in a medium saucepan. Stir in the flour and cook over moderate heat until golden, 1 minute. Gradually whisk in the milk until smooth. Bring to a boil over moderately high heat; whisking constantly, until thickened. Simmer the white sauce over low heat for 25 minutes, whisking often. Season with salt, pepper and the nutmeg. Cover and remove from the heat.
Preheat the oven to 350°. Butter a 9-by-13-inch glass baking dish. In a large skillet, heat the vegetable oil. Add the onion and tomato and cook over moderately low heat, stirring occasionally, until the onion is softened, about 8 minutes. Stir in the shredded chicken, Worcestershire sauce and rosemary; season with salt and pepper.
Stir the egg yolks into the white sauce. In a medium stainless-steel bowl, beat the egg whites until stiff. In a large bowl, combine the rice with the white sauce, raisins and cheese and season with salt and pepper. Fold in the beaten egg whites. Spread half of the rice in the prepared baking dish, cover with the chicken sauce and spread the remaining rice on top.
Bake the torta for about 40 minutes, or until piping hot throughout. Preheat the broiler. Broil the torta for 2 minutes, or until browned. Let stand for 5 minutes; serve with Ají and brown sugar.