In a small saucepan, combine the chicken stock, light soy sauce, oyster sauce, Shaoxing wine and sugar. Bring to a gentle boil, then reduce to a simmer.
In a small bowl, mix the cornflour with a little cold water until smooth. Stir this into the simmering sauce and cook for 1–2 minutes, until thickened and glossy. Remove from the heat, stir in the sesame oil and keep warm.
In a bowl, beat the eggs with a pinch of salt and pepper. Set aside.
Heat a little oil in a non-stick frying pan over medium-high heat. Add the sliced broccoli and cook until lightly charred.
Add the mushrooms, Chinese cabbage, garlic, ginger and chilli. Cook for about 1 minute until fragrant.
Add the prawns, season lightly, and cook until they just begin to turn pink.
Pour the beaten eggs evenly over the filling and gently swirl the pan to distribute. After about 30 seconds, scatter over the bean sprouts. Reduce the heat to medium, cover with a lid and cook until the omelette is set and golden underneath.
Fold the omelette in half and transfer to a plate. Spoon over the warm soy gravy and garnish with sliced spring onions, sesame seeds and extra bean sprouts.