Heat oven to 220C/200C fan/gas 7. Cut the potato and swede into equal- sized chunks, drizzle with 2 tbsp of the oil, season and roast for 50 mins or until completely soft. Leave to cool slightly, then pulse in a food processor until broken down. Add the chilli flakes, flour, Parmesan and some seasoning, then pulse again to form a sticky dough.
Bring a large pan of salted water to the boil in which to cook your gnocchi later. Flour your hands and divide the dough in three. On a floured surface, roll each portion into a sausage about 1cm in diameter. Using the back of a table knife, cut into pieces 2.5cm in length – this gives a tapered edge to the pieces of gnocchi.
Working in batches, cook the gnocchi in the water for 30 secs or until they rise to the surface. Remove with a slotted spoon and drain on a tray lined with kitchen paper. Can be made to this stage up to 4 hrs ahead and kept in the fridge.
Turn the oven to its lowest setting and heat the remaining oil in a large frying pan over a medium heat. Fry the gnocchi in batches for 2 mins on each side until browned, then keep warm in the oven.
Melt the butter in the pan, add the sage leaves and fry until crisp. Divide the gnocchi between bowls then spoon over the crisped sage and buttery sauce. Serve with pepper and grated Parmesan.