Bring a large pot of water to a boil and cook the soba noodles according to package directions. Drain and set aside.
Heat the sesame oil in the same pot over medium heat. Add the garlic, ginger, and turmeric. Stir for 1 minute until fragrant.
Pour in the vegetable broth and bring to a gentle simmer. Cook for 5 minutes to let the flavors blend.
Add the halved bok choy to the pot and simmer for 3 minutes until the stems soften slightly but the leaves remain bright.
Place the miso paste in a small bowl. Ladle about 1 cup of hot broth into the bowl and whisk until smooth. Stir the mixture back into the pot.
Divide the cooked noodles among four bowls. Ladle the broth and bok choy over the noodles.
Top each bowl with sliced green onions and sesame seeds before serving.