Heat the oil in a frypan over medium heat. Add the onion and garlic; cook and stir for a few minutes then add the green beans. Stir to coat with the flavours in the pan.
Season with soy sauce and fish sauce and let simmer for about 2 minutes, stirring occasionally. Pour in the water and let simmer for about 10 minutes, or until the beans are tender. Stir in tomato and season with salt and pepper before serving.