Cook the potatoes in a large pan of salted water with the lid on for approximately 30 minutes and drain once cooked.Whilst the potatoes are still warm, press them either through a sieve or a potato ricer twice. Alternatively with a good hand blender, blend until smooth.

Melt the butter in a saucepan, add the nutmeg and 2 level teaspoons of salt. Add the melted butter to the potatoes, lemon zest, egg yolks, semolina, flour and corn flour and mix to form a smooth dough.

Dust a work surface with a little polenta. Divide the dough into 4 portions and roll each portion into an even 1.5cm thickness, approximately 30cm long. Make sure there are no cracks and work with flat hands. Divide each roll into 2-3cm wide pieces with a dough scraper or a knife.

Before working with the gnocchi, dust your palms with a little polenta so that the dough does not stick. Press in grooves with the back of a fork and place the gnocchi on a smooth chopping board.

Carefully slide the gnocchi into boiling salted water, bring to the boil and then lower the heat. As soon as the gnocchi floats on the surface, let them steep for another 1-2 minutes. Then, remove the gnocchi with a slotted spoon.

To make the sage butter, heat the butter gently in a small pan then add the sage leaves and swirl gently. Pour the sage butter over the gnocchi in the square dish and serve immediately, sprinkled with parmesan.