Place 2 boneless, skinless chicken breasts in a medium pot, cover with water, bring to a boil, then reduce heat and simmer 15–20 minutes until cooked through.
Remove chicken from the pot, reserve 1 cup of cooking liquid, let chicken cool slightly, then shred with two forks.
Tear 5 slices white sandwich bread into pieces, place in a bowl, pour 1 cup evaporated milk over the bread, and let soak 5 minutes until softened.
Heat 1 tablespoon olive oil in a large skillet over medium heat. Add 1 large yellow onion and cook 5–7 minutes until translucent. Add 4 cloves garlic and cook 1 minute until fragrant.
Stir in 1/2 cup aji amarillo paste, 1 teaspoon ground turmeric, and 1/2 teaspoon ground cumin. Cook 2–3 minutes, stirring constantly.
Transfer the onion–aji mixture to a blender. Add the soaked bread and milk mixture plus 1 cup reserved chicken liquid (or broth). Blend until smooth.
Pour the blended sauce back into the skillet, bring to a gentle simmer over medium–low, stirring to prevent sticking. Add shredded chicken, 1/2 cup grated Parmesan cheese, and 1/4 cup chopped walnuts, and stir to combine.
Simmer 5–10 minutes until sauce thickens. Season with salt and freshly ground black pepper to taste. To serve, ladle stew over cooked white rice, and garnish with halved hard-boiled eggs and black olives.