Fill a medium-large pot halfway with water and bring to a boil. Add about 1 tablespoon salt and chicken breasts (if using). Return the water to a boil, then reduce to maintain a simmer for about 10 minutes, or until the chicken is cooked through (timing will depend on how large the breasts are, see recipe note). Transfer chicken to your cutting board and shred or chop it up to add to the noodles later.
Add the noodles to the boiling water and cook according to the package instructions. Drain and return to the pot.
In a medium bowl, whisk together the peanut butter, soy sauce, lime, miso, sesame, maple, and garlic until smooth. Add the water and whisk until smooth. Add more water if the mixture is too thick (it should be thick but drizzle-able, not pasty).
Transfer to the pot with the warm noodles and toss to coat. Add the veggies, ½ of the scallions, and chicken and toss again. If the sauce seems too thick and pasty, add a bit of boiling/hot water to thin it out. Keep in mind that if you are using cucumber, the water from the cucumbers will water down the sauce a bit as it sits.
Serve topped with sesame seeds or peanuts, more scallions, and sriracha (if using).