Cut tomatoes and red onion in half vertically. Cut each half horizontally into thin slices. Set aside.
Combine cilantro and next 8 ingredients (cilantro through sesame oil) in a large bowl; stir well. Add tomatoes, red and green onions, and bell pepper; toss well. Let stand 1 hour.
Note: Fish sauce is a salty condiment that can be found in Asian markets, specialty shops, and some supermarkets.