Heat half the oil in a wok or large frying pan, add the onion and stir-fry for 4-5 minutes, until starting to brown.
Add the garlic and mushrooms and stir-fry for a further 4-5 minutes, until the mushrooms have cooked through and are turning golden at the edges.
Spoon the mushrooms and onions into a bowl and set aside.
Heat the remaining oil in the wok. Add the steak and stir-fry for 2-3 minutes, until browned.
Add the spinach and cook for 1-2 minutes, until the spinach has wilted, then add the cooked mushrooms and onions, stock, oyster sauce, sugar and cornflour mixture, and bring up to a boil, stirring continuously.
Simmer for 1-2 minutes until the sauce has thickened and serve with rice or pasta.
To finish off you could sprinkle over some sliced spring onion.
Want dinner in less than 30 mins? This recipe is quick, easy & packed full of iron for growing families.