Bring a saucepan of salted water to the boil over medium heat. Add the gnocchi (drop into the base of the pan in a single layer, but don't overcrowd as they can stick together) and cook until they rise to the surface. Drain and transfer to a bowl. Drizzle over some olive oil and cover with cling film to keep warm.
Dip the cheese in ice-cold water and then into flour so that you cover all sides. Repeat and then shake off excess flour. Heat a little olive oil in a large frying pan and fry the cheese until golden on all sides, turning as necessary.
Rub about 30 sage leaves in your hand to release the flavour, then fry for a few seconds until they darken slightly. Lift out and drain on kitchen paper.
Put the kefalotyri saganaki and gnocchi in a serving bowl and scatter over the sage. Drizzle over a little melted butter and season with salt and pepper to taste. Serve immediately with a squeeze of lemon juice.