Bring a small pot of water to a boil and cook the egg for 6.5 minutes, then immediately transfer to an ice bath to cool.
In the same pot, cook the soba noodles according to package directions, drain, and set aside.
Slice the pork belly into thin strips and sear in a hot skillet over medium-high heat until the edges are golden and crispy.
In a medium saucepan, sauté the sliced mushrooms, minced ginger, and minced garlic in sesame oil until fragrant.
Pour in the beef bone broth and tamari, bringing the liquid to a gentle simmer for 5 minutes.
Add the baby bok choy to the broth and cook for 2 minutes until wilted but still bright green.
Divide noodles into a bowl, pour over the hot broth and vegetables, and top with the crispy pork belly, halved egg, and sliced green onion.