In a pot, bring water to a boil and cook the soba noodles according to the package instructions. Drain and set aside.
In a large skillet, heat the sesame oil over medium heat. Add the chicken breasts and cook until browned and cooked through, about 5-7 minutes per side. Remove from the skillet and let rest for a few minutes. Slice the chicken into thin strips.
In the same skillet, add the sliced green onions and sauté for 2-3 minutes until softened.
Add the chicken broth, soy sauce, mirin, grated ginger, minced garlic, and chili flakes to the skillet. Bring to a simmer and cook for 10 minutes.
Add the cooked soba noodles and sliced chicken to the skillet. Stir to combine and cook for another 2-3 minutes until heated through.
Season with salt and pepper to taste.
Serve the Negi Tori Soba in bowls, garnished with toasted sesame seeds and nori (seaweed). Enjoy!