The sweet and sour sauce most British people grew up with is not an ancient recipe. It is ketchup, vinegar and sugar, thickened with cornflour, and knowing that makes it something you can build from a cupboard in ten minutes.
Drain the pineapple and keep the juice. It is the liquid for the sauce, and throwing it away and using water is the single most common way to make this taste flat.
- Cube the luncheon meat, about 2 cm. Oil in the pan, good heat, and fry the cubes without crowding them until they are properly browned on at least two faces. This takes longer than you think, about six minutes, and it is where the dish is won.
- Lift the meat out. Dried onion into the same pan for thirty seconds.
- Mix the sauce cold, in a mug: ketchup, vinegar, sugar, soy sauce, cornflour and about 150 ml of the reserved pineapple juice. Cornflour stirred into hot liquid goes lumpy; stirred into cold liquid it never does.
- Pour it in, bring to a simmer, and it will thicken and turn glossy in under a minute.
- Pineapple chunks and the meat back in, just to heat through.
Taste it. If it is cloying, more vinegar. If it is sharp, a pinch more sugar. The balance point is personal and moves with the tin of pineapple.
It should coat a spoon and slide off it slowly. If it is gluey you used too much cornflour, and a splash more juice will fix it.