Use a potato peeler to pare off the potato skins as thinly as possible, then cut the potatoes into even-sized chunks – not too small; if they are large, quarter them, and if they are small, halve them.
Put the potato chunks in a steamer fitted over a large pan of boiling water, sprinkle salt all over them, put a lid on and steam the potatoes until they are absolutely tender – they should take about 25 minutes. The way to tell whether they are ready is to pierce them with a skewer in the thickest part: they should not be hard in the centre, and you need to be careful here, because if they are slightly underdone you do get lumps. When the potatoes are completely cooked, remove them from the steamer, drain off the water, return them to a saucepan and cover with a clean tea cloth for about 4 minutes to absorb some of the steam, then add the butter, milk, crème fraîche, sage and spring onions.
When you first go in with the whisk, use a slow speed to break the potatoes up, then increase it to high and whip them up to a smooth, creamy, fluffy mass. Taste and, if they need it, season. Serve with sausages braised in cider and stir-fried spiced red cabbage.