Rinse the rice under cold water. Bring 2 cups (480-500 ml) water to a boil in a saucepan over medium-high heat. Add the rice, stir once, reduce heat to low, cover, and simmer for 15-18 minutes until the rice is tender and water is absorbed.
While the rice cooks, cut the chicken breast into thin slices. Bring 500 ml vegetable stock to a simmer over medium heat in a separate saucepan. Add the chicken slices and poach for 12-15 minutes until fully cooked (internal temperature 74°C / 165°F). Remove the chicken with a slotted spoon and set aside.
Prepare the vegetables: peel or wash as needed. Slice the mushrooms thinly. Cut the onion into quarters. Cut the bell pepper into wide strips. Slice the carrot into thin strips. Cut the spring onions diagonally.
If the cashews are salted, rinse them under cold water to remove excess salt.
Heat a small amount of soy oil in a wok or large pan over medium-high heat. Add the mushrooms, onion, bell pepper, and carrot. Stir-fry for 5-7 minutes until tender-crisp. Pour in the 120 ml soy sauce and add the 1 teaspoon sugar, stirring to combine.
Add the 150 ml vegetable stock to create a sauce. Add the spring onions and cashew nuts. Return the cooked chicken to the pan and warm through for 2-3 minutes, stirring gently. Serve immediately over the cooked rice.