Bring a pot of water to a boil and cook the soba noodles for 6 minutes until tender, then drain and rinse under cold water.
In a small saucepan, soft-boil the egg for 6 minutes and 30 seconds, then immediately transfer to an ice bath, peel, and slice in half.
Season the beef sirloin with the sea salt and black pepper, then sear in a hot skillet for 2 minutes per side until browned and set aside to rest.
In a medium pot, bring the beef bone broth, tamari, minced garlic, and grated ginger to a simmer over medium heat.
Add the shiitake mushrooms and baby bok choy to the broth, simmering for 3 minutes until the vegetables are vibrant and tender-crisp.
Stir the fresh lime juice into the broth to add a bright, zesty finish to the base.
Divide the noodles into a deep bowl, pour the hot broth and vegetables over them, and top with the sliced beef and soft-boiled egg.