Bring a medium pot of water to a boil. Meanwhile, thinly slice cucumber, radishes, and nectarine, and combine them in a small bowl. Season with a pinch of salt and let sit for 5 minutes to draw out excess moisture. Gently squeeze out any liquid and set aside.
Add the soba to the boiling water and cook according to the package directions, about 4 to 5 minutes. Using tongs, transfer the noodles to a colander and rinse under cold running water until completely cooled and no longer sticky. Add to a bowl with water and ice before serving.
Return the water to a gentle boil, then carefully lower in the egg and cook for 7 minutes for a jammy yolk (or longer if you prefer it firmer). Transfer the egg to cold water to cool, then peel and halve.
While the egg cooks, whisk together the soy sauce, rice vinegar, maple syrup, sesame oil, and miso, if using, in a small bowl.
Transfer the drained soba to a serving bowl. Top with the cucumber mixture and the egg halves. Spoon over the dressing, then garnish with scallion greens and toasted sesame seeds. Serve immediately.