Combine the soy and chilli sauce in a large bowl. Add mushrooms and stir to coat. Cover and set aside 15 minutes to absorb marinade if time permits.
Place cashews into wok and heat over high heat, tossing cashews until warmed through. Remove cashews to a plate. Add 2 tsp oil and swirl to coat wok. Add half the beef and stir-fry 1 minute until sealed, remove to a plate, repeat with oil and remaining beef.
Add remaining oil to the hot wok with onion and garlic and stir-fry 1-2 minutes until softened. Add the mushrooms and marinade in the bowl and stir-fry 3 minutes until mushrooms are almost tender. Add choy sum stems, stir-fry a further 1 minute.
Add the capsicum and return the beef and any juices, stir-fry 1-2 minutes until beef is warmed through. Shred the choy sum leaves, add to the wok with cashews and stir-fry to combine. Remove from the heat and serve with steamed Basmati rice.