Combine the beef with the marinade ingredients and mix well. Set aside for 5 minutes (or longer if you have time — see Chef's Tips).
In a separate bowl, combine the Mongolian sauce ingredients and mix well. Set aside.
Heat the oil in a large wok or frying pan over high heat. Add the onion and stir-fry for 2 minutes. Add the beef, spreading the strips out as much as possible. Let it sear for 1 minute before flipping, then stir-fry for another minute, until you've got some good colour on the beef. Add the ginger and garlic and stir-fry for 2 minutes.
Add the cooked noodles and the Mongolian sauce. Stir-fry until well combined and the sauce is thick and glossy. Toss through the spring onions. Remove from the heat.
Serve in 5 x 750ml meal prep containers. Garnish with spring onion.
Chef's Tips
The baking soda in the marinade is doing real work, not just sitting there — it's a classic Chinese cooking technique called velveting, which tenderizes the beef by raising its surface pH. If you have a spare 15–20 minutes, letting it sit that long (rather than just 5) will noticeably improve the texture; just don't go much longer than 30 minutes or it can start affecting the flavour.Spread the beef out in a single layer and let it sit undisturbed for that first minute, as written — crowding or stirring too early steams the meat instead of searing it, and searing is where the colour and flavour come from.Have the noodles and sauce measured and ready to go before the beef hits the pan — this whole dish moves fast from that point on, and there's no real pause to prep once the wok is hot.
Storage & Reheating
Refrigerate for up to 3 days, or freeze for up to 3 months, in an airtight container.To reheat, warm in a pan over medium heat with a small splash of water or stock to loosen the noodles, or microwave for 2–3 minutes, stirring halfway through. Thaw frozen portions in the fridge overnight before reheating.