Scallions, onion, tomato and cilantro chopped fine and left in vinegar, lime and oil with as much chilli as you want. Nothing is cooked. It is what goes on the table next to empanadas in Colombia, and it keeps for weeks in a jar in the fridge.
Chop everything fine and by hand. A blender turns this into a purée, and ají is meant to be a loose relish where you can see the pieces of scallion and tomato. It is the texture as much as the heat that makes it work spooned over an empanada. Three hundred grams of scallion is not a mistake. Cebolla larga is the base of this sauce and everything else is built around it, which is why it goes in by weight while the onion and tomatoes go in by number. The vinegar does two jobs: it seasons and it preserves. A jar in the fridge keeps for weeks and the flavour is better after a day than it is in the first hour, because the scallion softens and its edge comes down. How hot it should be is a matter of taste and there is no correct answer. Add the chilli gradually, taste after mixing, and remember that it will get slightly hotter as it sits.