Bring a pot of water to a boil and cook the soba noodles according to package directions, then drain and set aside.
Pat the tuna steak dry with a paper towel and season both sides with sea salt and black pepper.
Heat the avocado oil in a cast-iron skillet over high heat until shimmering.
Sear the tuna for about 1-2 minutes per side for medium-rare, then remove from the pan and let it rest.
In the same skillet, reduce heat to medium and add the toasted sesame oil, minced garlic, and grated ginger.
Sauté for 30 seconds until fragrant, then add the chopped baby bok choy and cook until slightly wilted.
Add the cooked soba noodles to the skillet along with the tamari and lime juice, tossing everything to coat thoroughly.
Slice the tuna into thin strips and serve on top of the noodles, garnished with sliced green onions.