Heat the oil in a saucepan over a medium heat. Tip in the small chopped onions and most of the sliced spring onions (reserving around 2 tbsp) and fry for 6-8 mins, until soft but not golden. Scatter in the potatoes and fry for 4 mins, to soften a little, then pour in the stock and bring to a boil. Reduce the heat to medium-low and simmer for 15 mins. Season to taste, then tip in the sweetcorn and pour in the milk. Cook for 3-4 mins until the corn is tender. At this point, you can serve it chunky or blitz using a hand blender to break it up a little. Divide between bowls, then scatter over the chives before serving.