In a large pot, boil water for pasta. Add spaghetti and stir to prevent sticking.Cook according to package instructions.
In a large bowl, whisk miso, sesame oil, brown sugar, lime juice and garlic. Asyou whisk, slowly stream in oil until sauce is smooth and thick. Whisk in 2tablespoons water.
Drain spaghetti and rinse with cold water. Shake off any excess water and add to bowlwith sauce. Toss to coat evenly. Add bell pepper, cucumbers, and green onions.
Serve in individual bowls. Top with sesame seeds, fresh limes and additional greenonions, if desired.
Pasta can be made several hours ahead. They tend tosoak up a lot of the sauce as they sit. If you want to make the day before,hold back half of sauce, cover, and chill. Toss with reserved sauce before serving.