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. As you whisk, slowly stream in oil until sauce is smooth and thick. Whisk in 2 tablespoons water.
Drain spaghetti and rinse with cold water. Shake off any excess water and add to bowl with sauce. Toss to coat evenly. Add bell pepper, cucumbers, and green onions.
Serve in individual bowls. Top with sesame seeds, fresh limes and additional green onions, if desired.