Prepare the Broth:

1. Place the chicken breast, water, sliced carrot, celery, quartered onion, and crushed garlic cloves in a large pot.
2. Bring to a boil over medium-high heat, then reduce to low and let it simmer for 25-30 minutes, skimming any foam that rises to the surface.
3. After the chicken has reached a safe internal temperature, transfer it from the pot to a plate and allow it to cool down completely before proceeding.
4. Strain the broth to remove the vegetables and set aside 1 liter for the soup.
5. Tear the cooked chicken breast into small, manageable chunks.

Make the Soup Base:

1. In a spacious soup pot, warm the olive oil over a medium flame.
2. Add the chopped onion and minced garlic, cooking until soft and translucent, about 3-4 minutes.
3. Add the sweet corn kernels, stirring well, and cook for another 2-3 minutes.

Combine and Cook:

1. Pour the prepared chicken broth into the pot with the vegetables and bring to a gentle boil.
2. Add the shredded chicken and soy sauce (if using) and season with salt and pepper to taste.
3. In a small bowl, mix the cornstarch with water until smooth, then add it to the soup to thicken.
4. Let the soup simmer for another 5-10 minutes, stirring occasionally until it reaches your desired thickness.

Serve:

1. Ladle the soup into bowls and garnish with chopped green onions and fresh cilantro, if desired.
2. Serve hot and enjoy the comforting flavors of this homemade soup.