1.Heat 2 teaspoons of the oil in wok; stir-fry beef, in batches, until browned. Remove from wok.
2.Heat another 2 teaspoons of the oil in wok; stir-fry mushrooms until browned lightly. Add garlic and green onion; stir-fry until fragrant. Return beef to wok with oyster sauce; stir-fry until hot, season to taste. Remove from wok; cover to keep warm.
3.Whisk eggs with the water and soy sauce in large jug. Heat 1 teaspoon of the oil in wok; pour in a quarter of the egg mixture, tilting the wok to make a 20cm (8-inch) omelette, cook until almost set. Slide omelette onto serving plate; top with a quarter of the beef mixture and a quarter of the spinach. Fold omelette to enclose filling; cover to keep warm.
4.Repeat to make a total of four omelettes.