For the sauce, combine the chicken stock, oyster sauce, soy sauce and Shaoxing wine in a small saucepan over high heat. Simmer for 2 minutes. Stir through the cornflour mixture, then simmer for another minute until the sauce has slightly thickened. Turn the heat off and stir through the sesame oil. Remove pan from heat and set aside for later.
Heat a non-stick frying pan or wok over high heat. Add 1 tablespoon of vegetable oil, followed by the pork and prawns. Stir-fry for 2 minutes or until just cooked. Transfer to a plate and allow to cool completely.
Meanwhile, in a large bowl, whisk the eggs with the cornflour and 1 teaspoon of the vegetable oil. Add the cooled pork and prawns and spring onion.
Place a wok or frying pan over medium-high heat. Pour in the remaining 3 tablespoons of oil. Wait a few seconds for the oil to heat up, then pour in the egg mixture. The egg should start to set and puff at the edges immediately. Once the egg is almost set in the middle. Then flip the omelette over (you may have to do this in pieces). Cook until just set.
Scoop out chunks of the omelette onto a serving plate. Reheat the sauce (add a little water if it is too thick) and pour over the omelette. Serve immediately.