Cover the chicken with water in a pot and simmer gently until cooked through, 15 to 20 minutes. Reserve 1 cup of the cooking liquid, cool the chicken, and shred it.
Blend the bread and evaporated milk until smooth.
Cook the Sauce:
Cook the onion and garlic in a skillet with a little oil until soft, about 6 minutes. Stir in the ají amarillo paste and cook for 1 minute.
Add the bread mixture and enough reserved stock to make a thick sauce. Simmer, stirring, until creamy, about 8 minutes.
Fold in the chicken, walnuts, and Parmesan. Warm gently and season to taste.
Serve:
Spoon the ají de gallina over warm rice. Garnish with sliced egg and olives.