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1
Place the spring onion greens and the oil together in a wok or heavy saucepan. Begin over low heat. The oil and onions must start cold together — do not add the onions to hot oil. Bring the temperature slowly to 150 °C over 20–30 minutes. The onions will first soften, then blister gently. The oil is ready to proceed when it holds a steady, quiet bubble around each strand of onion.
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2
Continue at 150 °C, watching constantly. The spring onions will turn deep golden and begin to crisp at the edges. Remove them from the oil the moment they reach a deep, even gold — this is where the dish lives or dies: the onions continue to colour in the residual heat of the oil after removal, and a single inattentive minute carries them from caramelised sweetness to bitterness. Lift them out with a slotted spoon onto a plate.
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3
Strain the scallion oil through a fine sieve into a clean jar or bowl. The oil should be a clear, deep amber-gold, fragrant with caramelised onion. Reserve the fried spring onions separately. The oil may be kept at room temperature for up to two weeks.
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4
Combine the light soy sauce and dark soy sauce in a small bowl. Stir briefly. The sauce is done: it should smell sharp and clean with a deep, dark colour from the dark soy.
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5
Cook the fresh egg noodles in a large pot of unsalted boiling water according to their thickness — fresh thin noodles require only 1–2 minutes. They are done when tender but still springy to the bite; drain immediately.
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6
Divide the hot noodles among four bowls. Spoon several tablespoons of scallion oil over each portion, followed by the soy sauce mixture. Toss at the table or in the bowl until every strand is glossy and uniformly amber-coated. Scatter the reserved fried spring onions over the top. The dish is ready when no pale, undressed noodle remains visible.