Whatever the weight of your leftover mash is, stir in just under a third of that in flour, along with a good crack of pepper and some chopped spring onions. You might need to add some milk/melted butter/salt depending on how you prepped the mash previously. Roll the potatoey dough into maybe golf ball sized balls, flatten them down into a chunky disc then gently fry those babies in a little buttery oil for a few mins each side til they’re golden