Warm 400ml veg oil in a small pan over low-medium heat until just shimmering, then add the thinly sliced shallots and fry, stirring often, for 15-20 minutes until golden. Remove the shallots from the oil using a slotted spoon (reserve the oil for later use) and transfer to a plate lined with kitchen paper.

Season the shallots with salt and store in a sealed container for up to 5 days.