Heat olive oil in a soup pot over medium heat. Add sliced chicken and cook until lightly browned and just cooked through. Remove chicken and set aside.
In the same pot, add garlic and ginger. Sauté for about 30 seconds until fragrant.
Pour in chicken broth and bring to a gentle simmer.
In a small bowl, whisk miso paste with a few tablespoons of warm broth until smooth. Lower the heat and stir the miso mixture into the soup.
Return the cooked chicken to the pot. Add soy sauce and spinach or bok choy if using. Simmer gently for 3–5 minutes until greens are wilted.
Drizzle with sesame oil, taste, and adjust seasoning if needed. Serve warm topped with green onions.
