Make the marinade: in a small bowl use a small whisk to combine the marinade ingredients and set aside.
Make the stir fry sauce: in a small bowl use a small whisk to combine the stir fry sauce ingredients and set beside your stove.
Marinate the beef: due to the shape of beef steaks the pieces will be a mix of sizes; separate them into two lidded containers in batches of big and small pieces. Divide the marinade between containers, stir through to ensure the pieces are fully coated, put the lids on, and refrigerate for 30 minutes.
Prepare the noodles: follow the packet instructions, which usually call for soaking the noodles until tender and transparent.
While the beef is marinating and noodles soaking, stir fry all the vegetables except the mushrooms: to the wok, over high heat, add the vegetable oil, heat until shimmering, add the garlic, onion and capsicums, stir fry for a few minutes (or a little longer if you prefer), then transfer to a bowl and cover. Reduce the heat to medium and if needed wipe out the wok with a paper kitchen towel.
Finish the noodles: drain the noodles in a fine mesh strainer. To the same wok, over medium, add a tablespoon each of sesame oil and oyster sauce and the noodles, stir fry for a minute or two until any excess water has evaporated and the noodles are browned by the sauce and coated in sesame oil, and transfer the noodles to a bowl and cover. Use a paper kitchen towel to wipe the wok removing any noodles that stuck.
Stir fry the beef in batches: increase the heat to high, when the wok is very hot and just starting to smoke, add a tablespoon of vegetable oil, swirl to coat the wok, and add the batch of bigger beef pieces. Use a spatula to separate the pieces, let the beef piece sit and sear for a minute or so to colour and even char a little, then stir fry the beef for another minute or so; let the pieces sit and sear again for another minute or so, then transfer the pieces to a bowl to rest. Repeat with the second batch of smaller beef pieces – note that they’ll cook faster – then transfer them to the bowl.
Stir fry the mushrooms: add a tablespoon of vegetable oil, use the spatula to scrape any meaty pieces off the wok and combine with the oil, add the mushrooms, and stir-fry in the brown beefy oil over high heat for a few minutes until brown, then transfer the mushrooms to the bowl of vegetables.
Stir-fry the lot: remove the wok from the heat, reduce the heat to medium, and use a paper kitchen towel to wipe out the wok. Add the stir-fry sauce to the wok, return the vegetables and beef to the work, stir fry until well combined and fully coated in the sauce. Add the noodles and stir to combine well with the vegetables and beef, sprinkle on the sesame seeds and roasted peanuts, and stir to combine. Transfer to a serving plate and serve immediately.
Garnish with more sesame seeds and serve with dishes of crushed peanuts and fresh coriander or sliced spring onions for guests to add as they like.