Heat two teaspoons of oil in a large wok or frying pan, then add the beans and cook over a high heat for five minutes, to defrost. Add the onion and pepper and fry for three to five minutes more, until the vegetables start to soften. Tip onto a plate.
Add the remaining oil to the pan, then fry the chicken for three minutes, stirring continuously, until browned on the outside. Add two tablespoons of cold water, then cover the pan and simmer for five to seven minutes until the meat is cooked through.
Stir in the pineapple and the grains, then return the vegetables to the pan. Stir-fry until piping hot. Splash in the soy sauce (if using), toss well, then serve.