Vietnamese scallion oil: hot fat poured over sliced spring onion so that it hisses and sets bright green
Origin: Vietnam
From the journey of Spring Onion.
This is barely a recipe and it is on half the plates in Vietnam. Sliced spring onion goes into a bowl. Oil is heated until it is genuinely hot, hot enough to shimmer and move, and poured straight over. The onion hisses, wilts, and fixes at a bright green that raw onion never has and cooked onion loses. Twenty seconds, two ingredients, and the country's most-used condiment is made. The technique is worth understanding because the timing is the whole of it. Spring onion held in oil over heat will keep cooking and go khaki and bitter, which is the Shanghai method and produces something quite different and also good. The Vietnamese method takes the pan off the heat and brings the oil to the onion instead, so that the onion receives one brief burst and then cools. The chlorophyll sets, the harsh sulphur compounds are knocked back, and the flavour that transfers to the oil is fresh rather than caramelised. Where it goes is everywhere. Over cơm tấm, the broken rice with grilled pork chop that is Saigon's breakfast; over grilled corn from a street cart; over bánh of many kinds; over grilled prawns, oysters, snails, and clams; over plain rice when there is nothing else. In the south it is often enriched with a little pork fat rather than made with oil alone, which is better still. A pinch of sugar and a splash of fish sauce are common additions and both are optional. What is not optional is using enough onion.
A hundred and fifty grams of spring onion to a hundred and twenty millilitres of oil is a thick, spoonable condiment rather than a flavoured oil. That is correct: mỡ hành should be scooped, not drizzled. It keeps for four or five days in a jar in the refrigerator, though the colour dulls after the first day. Made fresh it is noticeably better and takes ten minutes. The pork fat version is the traditional southern one and is worth doing when you have some. Render it from pork belly trimmings and use it for half the oil. Do not put the onion in the pan and heat it. That gives the Shanghai cōng yóu, which is a fine thing but a completely different one, brown and deep instead of green and fresh. Serve over cơm tấm (broken rice with grilled pork chop), over grilled sweetcorn, or over grilled oysters with crushed peanuts, which is how it is eaten on the coast.
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