Again, but this time with the leftover potato and celeriac mash.
Peel, slice and fry two slim leeks: equivalent to, but containing more flavour than one giant leek. Fry the leek in butter with a splash of oil to stop in burning. You want a heat low enough so that it takes about 15 minutes for them to get lightly browned and sticky.
Add about a litre of stock, making sure you incorporate any gooey bits on the bottom of the pan from the leeks.
Then, about 500g of leftover mash, and bring it up to the boil.
Salt and pepper, and if in doubt, a bay leaf or two won’t hurt.
Let it simmer for a bit, so the starch in the spuds is unleashed and can thicken the soup. (A quick attack with the hand blender if, like me, you haven’t chopped the leeks finely enough or there are lumps in the mash. One need never fear making soup mid week with one of these in the cupboard.)
Once that’s all done, and the heat is off, you could crumble a small amount of Stilton or Gorgonzola into the soup, stirring until it has melted. Or just serve with a dollop of crème fraîche.
Then sit back and enjoy the comforting starchy goodness of it all.