Trim the roots and any limp tips from the 200–220g of scallions, then cut the white and pale green parts into 5–6cm / 2–2½ in lengths. Cut the remaining greener sections into similar longer batons for frying, and thinly slice a small handful of the freshest green tops for garnish. Pat everything very dry with a clean towel, paying attention to the cut ends where water collects. That thorough drying is what keeps the oil calm and lets the batons dehydrate and caramelise instead of steaming and splattering.

Pour the 180ml of neutral oil into a wok or saucepan and add the scallion batons while the oil is still cold or only just warming. Set the pan over medium to medium-low heat and cook slowly, stirring occasionally, for about 12–18 minutes depending on your burner, until the scallions turn deep golden to dark golden brown, softened, and lightly crisp at the edges. If the oil begins to bubble hard, lower the heat. Lift the scallions out with a strainer or slotted spoon just before they seem too dark, and spread them on a plate. That slow dehydration is what builds the flavour of the entire dish.

Leave the scallion oil in the pan, take the pan off the heat for a moment, and let the oil settle. Return it to low heat, then add the 4 tbsp light soy sauce, 2 tbsp dark soy sauce, 2½ tbsp sugar, and the 1 tbsp oyster sauce if using, standing back slightly because the liquid sputters on contact with the hot oil. Simmer gently for 1–2 minutes, just until the sugar dissolves and the sauce turns glossy and unified, then turn off the heat. That short, gentle simmer is what dissolves the sugar without tightening the sauce, so it stays fluid enough to coat every strand.

Bring a large pot of water to a rolling boil and cook the noodles until just tender, which takes about 1–3 minutes for the 600g of fresh noodles, or 4–6 minutes for the 400g of dried noodles, following the package directions. Before draining, scoop out a few tablespoons of the hot cooking water and set it aside. Drain the noodles very well, shaking the colander so no water clings. The strands should stay springy and silky, never soft. That thorough draining is what keeps the sauce from being diluted.

While the noodles are still hot, tip them into a large bowl or back into the warm pot and pour over most of the scallion oil sauce. Toss thoroughly with chopsticks or tongs until every strand is evenly coated. Add 2–3 tbsp of the reserved noodle water only if needed to loosen the noodles and help the sauce spread, then taste and add a little more sauce if you want more depth. The finished noodles should look glossy and evenly amber, not wet or soupy. That toss while hot is what lets the warm, starchy strands absorb the flavour instead of just wearing it.

Divide the noodles among 4 bowls, mounding each into a generous tangle. Top every bowl with a share of the fried scallion strands, then place a small pinch of the finely sliced fresh green onion in the centre. Serve immediately, while the noodles are hot and glossy and the scallion strands are still crisp at the edges. Skip any extra vegetables, protein, or sauce on the side. That restraint is what keeps the scallion oil as the single, unmistakable flavour in the bowl.