Pour 2 liters of water into a deep saucepan and set over medium-high heat. Cover and bring to a simmer. Dice the chicken into thin strips, set aside. Place the miso paste into a small bowl and dilute it with a couple of tablespoons of water from the saucepan and stir to a loose consistency. Add the miso, chili powder, and tamari soy sauce to the boiling water along with the chicken. Poach the chicken for 10-12 minutes to cook through, but to also flavor the broth.
Add the tahini into a large jug then add ½ cup of the hot broth, whisk thoroughly to combine. It will first thicken into a dense paste, but keep with it and add another ½ cup of hot broth. The paste will begin to loosen up. Repeat a final time until the tahini is a loose but smooth consistency. Pour the tahini mixture in a slow stream back into the main broth pot and thoroughly whisk through. Taste and add a pinch of sea salt or more tamari soy sauce if it needs it. Dice the oyster mushrooms and add them to the broth and simmer for 5 minutes. Drop in the beansprouts and simmer for another 2-3 minutes.
Divide the chicken, mushrooms, and beansprouts between bowls and ladle over the ramen broth. Garnish each bowl with more fresh green tip scallions, sesame seeds, chili flakes, or nori seaweed before serving.