Bake the potatoes. Then, as soon as they’re cooked, remove them from the oven, leaving the heat at gas mark 6, 400°F (200°C), then (with your hands in oven gloves, or using a tea towel for protection), cut the potatoes in half and scoop out the centres into a bowl. Next arrange the potato shells in a shallow roasting tin (using one potato to prop another up if they don’t sit comfortably straight).
Now melt the butter in a saucepan and fry the spring onions for 2-3 minutes; then add all the onion and melted butter to the potato together with the soured cream and egg yolks, and whisk (preferably with an electric hand whisk), until smooth.
Taste and season well with salt and pepper and add the parsley. Now, with a clean whisk, beat the egg whites until stiff and carefully fold them into the potato mixture. Pile the mixture back into the potato shells and return them to the top half of the oven for 15-20 minutes, or until puffed and golden brown on top.
Serve immediately – with some home-made chutney to dip the skins in.