Bring a large pot of water to a boil over high heat and salt it, if desired.
Meanwhile, in a blender, preferably a high-speed one, combine the spinach, cilantro, tahini, lime juice, water, scallions, garlic, soy sauce, and maple syrup or honey. Blend until combined and flecks of green splatter the interior. Stop the blender to scrape down the pitcher. If the sauce is too thick, add more water, 1 tablespoon at a time, and continue blending until a smooth, green sauce forms. Taste, and add more lime juice, if desired.
Add the soba to the boiling water and give the noodles a quick stir. Boil until just tender, about 3 minutes. Drain, transfer to a large bowl and immediately toss with the sesame oil until well coated.
Just before serving, divide the noodles among plates or shallow bowls and drizzle with the sauce. Serve warm, with sesame seeds sprinkled over the top, if desired.
