1.Heat 2 teaspoons of the oil in a wok over high heat; stir-fry beef, in batches, until browned. Remove from wok.
2.Heat another 2 teaspoons of the oil in wok; stir-fry onion and garlic until fragrant. Return beef to wok with oyster sauce; stir-fry until hot. Remove from wok; cover to keep warm.
3.Whisk eggs with fish and soy sauces in a large jug. Heat 1 teaspoon of the oil in same wok; pour in a ¼ cup of the egg mixture, tilting the wok to make a 14cm omelette, cook until almost set. Slide omelette onto a serving plate; cover to keep warm. Repeat to make a total of eight omelettes.
4.Fill omelettes with beef, mushrooms, herbs, sprouts and chilli.
To make a ‘net-style’ omelette, place the egg mixture in a plastic zip-lock bag. Snip a small hole in one corner and drizzle the mixture into a heated oiled wok.