Chop the chicken breasts into small 1/4-inch pieces. In a large bowl, whisk together the egg and cornflour. Add the chicken and stir to coat.
Heat the oil in a large, high-rimmed skillet over medium-high heat. Add the chicken and fry, stirring frequently. Once the chicken is starting to color, transfer to a bowl using a slotted spoon. Set aside.
In the same skillet set over medium-high heat, fry the onion and garlic until translucent and fragrant, about 5 minutes. Add a touch more oil if the skillet is too dry. Add the chili and ginger and continue to cook for another couple minutes. Stir in the oyster sauce and soy sauce. Once evenly combined, return the chicken to the pan and add the stock and chopped cashews. Bring to a gentle simmer and turn the heat to low. Continue to simmer for approximately 10 minutes or until the chicken is cooked through and the liquid has reduced to a coating consistency. Taste for seasoning and add salt if desired.
Separate the leaves of lettuce carefully, keeping each leaf in one piece. Wash and dry.
If serving with rice, place a spoonful of rice in each lettuce leaf. Top with the chicken and a sprinkle of finely chopped spring onion. Enjoy while warm.