Bring a large pot of salted water to a boil. Cook noodles according to package directions until just tender, usually 6-8 minutes for Chinese noodles or 10-12 minutes for spaghetti. Drain immediately and rinse under cold running water until completely cool, tossing with your hands to prevent sticking. Drain well and transfer to a large bowl. Drizzle with sesame oil and toss to coat. This stops clumping and adds subtle flavor.

In a medium bowl, whisk together peanut butter, soy sauce, rice vinegar, lime juice, honey, garlic, and ginger until smooth. The mixture will be thick at first. Add warm water 1 tablespoon at a time, whisking until the sauce is pourable but still coats the back of a spoon. Taste and adjust - more lime for brightness, more honey for sweetness, more soy for salt. You want a bold, balanced flavor that will mellow slightly when chilled.

While noodles cool, prep your vegetables. Slice the bell pepper into thin strips about 2 inches long. Julienne the cucumber into matchsticks, removing seeds if watery. Shred carrots on the large holes of a box grater or cut into fine matchsticks. Thinly slice the scallions, keeping white and green parts in separate piles. The vegetables should be crisp and uniform so they distribute evenly and eat well cold.

Add the bell pepper, cucumber, carrots, and white parts of scallions to the cooled noodles. Pour about three-quarters of the peanut sauce over the top. Use tongs to toss thoroughly, lifting from the bottom to ensure every strand is coated. The noodles will absorb sauce as they sit, so go slightly heavier than you think. Reserve remaining sauce for serving.

Cover and refrigerate for at least 30 minutes, or up to 4 hours. Cold temperatures firm the noodles and let flavors meld. Before serving, toss again and add a splash of reserved sauce if the salad looks dry. Transfer to a serving bowl or individual plates. Top with green scallions, chopped peanuts, cilantro, and sesame seeds if using. Serve cold with lime wedges on the side.