Add the olive oil to a deep fry pan, or your wok over medium high heat. Toss in the onion and the white parts of the bok choy. Stir frequently, to prevent burning, until the onion starts to caramelize. The bok choy should still be crunchy. Add the chopped bok choy leaves in a pile on top and stir them in, along with the fresh ginger, black pepper, tofu, and crushed red pepper. (You may substitute cayenne pepper for the crushed red pepper, but add it to the soup bowl in the last step. Add the chicken and vegetable broth and stir again. Add the soba noodles and stir. Lower the heat to medium, cover and steam cook for about four minutes. If I am planning to eat this for more than one meal, I take it off the heat before the noodles are completely done. In the bottom of each serving bowl, add 1/4 teaspoon dark sesame oil, and here is where you would add the cayenne pepper, if you are using it. I like it very spicy, so I add about 1/8 - 1/4 teaspoon. Ladle the soup into each bowl and serve. This recipe is has very little salt in it, so you may want to add some crushed sea salt, or a bit of dark soy sauce to enhance the taste.