If you’re using leftover baked potatoes, skip these first 3 steps. Preheat the oven to 180ºC/gas 4. Wash the potatoes and dry well, then prick each one with a fork and wrap in foil.
Put on a rack in the middle of the oven and cook for about 1 hour 15 minutes. The exact time depends on the size of your potatoes, but to check, give them a quick squeeze, bearing in mind they’ll be hot: the potato should just give under your fingers.
Remove from the oven then, when cool enough to handle, cut them into quarters. Leave to cool completely.
Peel and dice the onion. Melt the butter in a large saucepan over a medium-low heat and cook the onion for 10 minutes, or until soft.
Cut the potatoes, skin and all, into 2cm chunks. Add the potato and Parmesan rind to the pan, season, then cook for 5 minutes.
Add 1.25 litres of stock, bring to the boil, then reduce the heat and simmer for 30 minutes.
Remove the rind, then purée the soup until smooth with a stick blender or in a liquidiser. Return to the pan, check the seasoning, and thin with the remaining stock, if you like.
Serve garnished with a dollop of sour cream, snipped chives and a pinch of black pepper. Delicious with crumbled Lancashire cheese and crisp, crumbled bacon on the side for the whole baked potato experience.