Bring a large pot of unsalted water to a rolling boil. Add soba noodles and cook according to package directions, usually 6 to 8 minutes, until tender but still slightly firm to the bite. Stir once during cooking to prevent clumping.

Drain noodles in a colander and immediately rinse under cold running water, rubbing gently with your hands to remove excess starch. Continue until noodles are completely cold. Shake colander well to remove as much water as possible.

In a small bowl, whisk together tahini, soy sauce, rice vinegar, sesame oil, honey, garlic, and ginger until smooth. The mixture will thicken at first.

Add warm water 1 tablespoon at a time, whisking after each addition, until the sauce is pourable but still coats the back of a spoon. You may not need all the water depending on your tahini's thickness.

Transfer cooled soba to a large serving bowl. Pour sauce over noodles and toss thoroughly with tongs or two forks until every strand is evenly coated. The noodles should look glossy, not pooled with sauce at the bottom.

Sprinkle with toasted sesame seeds and sliced scallion greens. Serve immediately, or chill for up to 30 minutes for an even more refreshing dish.