If using fresh wide rice noodles, separate the strands gently by hand. If they are cold and stuck together, warm them very briefly in the microwave or over steam, just until they loosen without breaking. If using dried wide rice noodles, cook them according to the package until just tender, then drain well, rinsing only if needed to stop the cooking, and drain thoroughly again. You need 800g cooked noodles in total. If the strands begin to stick, toss them with 1–2 tsp neutral oil, using only as much as they genuinely need. The noodles should be pliable and fully separated before they reach the wok. That separation matters because clumped noodles tear, cook unevenly and never absorb the sauce properly.

Place the 800g sliced beef in a bowl and add the 1 tbsp light soy sauce, 1 tbsp oyster sauce, 1 tsp fish sauce, 1 tsp cornstarch, 1 tsp neutral oil and ¼ tsp pepper. Mix well until every slice is evenly coated and no dry patches of cornstarch remain visible. Leave it to stand for 15 minutes. The beef should look lightly glossy and clinging rather than sitting in a pool of liquid; if a little marinade gathers at the bottom of the bowl, stir it back through just before cooking. That short marinade is what seasons the beef and improves its browning, while the cornstarch and oil form a thin layer that genuinely protects the slices during the fierce sear.

In a small bowl, stir together the 3 tbsp oyster sauce, 2 tbsp light soy sauce, 1 tbsp dark soy sauce, 1½ tbsp fish sauce, 1½ tbsp brown or palm sugar and 2 tbsp of the water or low-sodium stock. Keep stirring until the sugar has dissolved completely and no grains settle at the bottom. Taste it: it should be savoury, salty and a little sweet, and slightly too strong on its own. If it feels too thick to pour easily, loosen it with the remaining 1 tbsp water or stock. Keep the dark soy to the measured amount, since it is there for colour and a little depth. That deliberately assertive seasoning is what lets one bowl of sauce flavour a full wok of noodles, beef and onion.

Halve the 2 onions and slice them into half-moons about 5mm thick so they soften quickly but keep their shape. Thinly slice the 4–8 red chillies, using fewer for a milder result, then slice the few extra chillies for serving and keep them separate. Finely chop the 6 garlic cloves and pick the 40–50g Thai basil leaves from their stems, setting the small handful of extra leaves aside for the garnish. Arrange the beef, noodles, sauce, oil and aromatics beside the stove in the order they will go into the wok. That complete setup matters because this is a very fast stir-fry, and once the wok is hot there is genuinely no time left to slice or measure anything.

Heat a wok or large heavy skillet over high heat until very hot and just beginning to smoke. Add 1 tbsp of the neutral oil and swirl it around the pan. Lay half the beef in a single layer and sear for 45–90 seconds, turning only as needed, until browned on the outside but still slightly pink inside. Transfer it to a plate, then repeat with another 1 tbsp oil and the remaining beef. If the slices release liquid and turn grey, the pan is crowded or not hot enough, so let it reheat fully before the next batch goes in. That brief, very hot sear is what browns the beef without overcooking it, since it finishes heating through later with the noodles.

Keep the wok over high heat and add the remaining 1 tbsp neutral oil. Add the garlic and sliced chillies and stir-fry for 10–15 seconds, just until fragrant and before the garlic takes on any real colour. Add the onions and cook for 1–2 minutes, stirring often, until they have softened slightly at the edges but still hold their shape and a little bite. They should look glossy and lightly softened rather than deeply caramelised. If the garlic starts to darken too quickly, add the onions at once to cool the pan. That short, hot stir-fry is what releases the garlic and chilli aroma into the oil while keeping the onions sweet and distinct.

Add the 800g noodles to the wok, then pour the sauce around the edges of the pan rather than directly onto one spot. Using tongs or a wok spatula, lift and toss the noodles gently until they are evenly coated and glossy. Between tosses, let them sit against the hot surface for a few seconds at a time so some edges pick up a little caramelised colour. Cook for 1–2 minutes in total. The noodles should absorb and hold the sauce, with nothing pooling at the bottom of the wok. If liquid collects, the heat is too low or the pan is overcrowded; with a small wok, toss the noodles in 2 batches, dividing the sauce between them. That short contact with the hot metal is what gives the noodles their glossy coating and lightly charred, genuinely stir-fried edges.

Return the seared beef and any juices from the plate to the wok. Toss for 30–60 seconds, until the beef is just heated through and evenly mixed through the noodles and onions. Add the 40–50g Thai basil leaves and toss just until they wilt, which takes only a few seconds. Remove the wok from the heat immediately. The basil should look bright and glossy rather than dark or slimy, and the beef should still be tender and juicy rather than dry. That last-moment addition is what keeps the Thai basil aromatic, because longer cooking dulls its flavour and darkens the leaves too much.

Divide the noodles between 4 bowls, about 200g of noodles and a quarter of the beef in each, with an even share of the onions and basil. Top each bowl with a few of the extra Thai basil leaves and the extra sliced red chillies. Serve immediately, while the noodles are still hot, glossy and slippery. The finished dish should be a coated stir-fry rather than a wet or soupy one, with tender beef and visible basil throughout. If the noodles look dry and stiff, they spent too long in the wok, so pull them sooner next time. That immediate serving is what keeps the noodles slippery and the basil fragrant, since both lose their appeal quickly as the dish cools.