Place a wok or frying pan over high heat until hot. Add oil, swirling to coat sides. Add diced onion and stir-fry for 1 minute. Add chicken and stir-fry for 1 minute
Stir in fish sauce, chili sauce, and sugar. Add eggs and stir-fry for 1 minute
Add basil leaves and green onion; cook for 30 seconds
To serve, line the buns with lettuce leaf, spoon chicken mixture into buns and tomato slices into each sandwich and serve