Slice chicken breast into long, thin strips; season with ¼ teaspoon salt, 1/8 teaspoon white pepper, corn starch and a drizzle of vegetable oil. Set aside.

In a soup pot, heat a tablespoon of vegetable oil over medium heat. Stir-fry light green parts of green onions until softened, about 1 minute.

Add sliced mushrooms and cook, stirring until mushroom has softened, about 3 minutes. Season with salt.

Add water and turn the heat to high. Bring the mixture to a boil; add seasoned chicken strips and 1 teaspoon soy sauce. Gently stir and let cook for 3 minutes.

Correct seasoning with salt and white pepper, if needed. Stir in spinach and remove from heat.

Ladle the soup into individual bowl. Serve with a drizzle of sesame oil and top with dark green parts of green onions.