Place chicken in a large pot and fill with water to halfway up sides of bird. Add leek, onion, carrot, tomato, salt, peppercorns and bay leaves. Cover, bring to a boil, reduce the heat and simmer for 45 minutes, or until chicken is done. Remove chicken and place in a bowl until cool enough to remove the meat from the bones and shred into bitesize pieces. Meanwhile, strain broth and reserve 4 cups.

Soak the bread in the milk. Heat oil in a large skillet over medium heat. Add the onions and saute until soft and golden, about 10 minutes. Add the garlic and cumin and saute for 1 more minute. Add softened bread and aji or chile powder, and cook for about 1 minute, stirring. (For a creamier sauce, place onion-bread mixture in a blender, puree until smooth and return to skillet.) Add 2 cups of the chicken stock and continue cooking until sauce thickens and coats the back of a spoon, about 5 minutes.

Add the chicken and simmer for 5 to 8 more minutes, adding more stock if needed. Stir in the walnuts and Parmesan cheese, and simmer gently for another 5 minutes, or until thickened slightly. To serve, spoon chicken and sauce onto a serving platter and surround with potatoes and eggs. Sprinkle with chopped parsley.