Bring a large pot of water to a boil. Cook spaghetti until just tender, 9 to 11 minutes or according to package directions. Drain; rinse under cold water.
Meanwhile, whisk soy sauce, sesame oil, canola oil, vinegar (or lime juice), crushed red pepper, 1/4 cup scallions and 2 tablespoons cilantro (if using). Add noodles, snow peas and bell pepper; toss to coat.
To serve, mix in sesame seeds and garnish with the remaining scallions and cilantro.