Bring a large pot of water to a boil. Add the soba noodles and cook according to the package instructions, adding the greens in during the last 1-2 minutes. (See “fun fact” and save some of the cooking water if you’d like). Drain and rinse with cold water until the noodles and greens are cold.
Remove the greens, squeeze out excess water, form them into a tight log, and thinly slice. To keep the noodles from sticking, drizzle some sesame oil onto the cold noodles and gently toss to coat.
In a medium bowl or jar, add the tahini, soy sauce, vinegar, sesame oil, and maple, and stir to combine. It will clump up and look weird for a bit, but keep stirring until it thickens, then stir in the water until totally smooth. You want it to be just barely thick enough to coat the noodles lightly, but not so thick that it glops onto the noodles. Add more water as needed.
In your bowl, add the noodles, greens, cucumber, and tofu (if using). Top with sesame seeds, scallions, and nori (if using). Add ¼ of the sauce to a small bowl to serve on the side. Use chopsticks (or a fork) and dip each bite of noodles and the tofu slabs into the sauce, letting the excess sauce drip into your bowl.
If you saved the buckwheat cooking water, pour it into your sauce cup (you should have a bit of sauce left in there), give it a stir, and drink up!