Heat an oven to Gas 6/200ºC/400ºF. Bake the potatoes for 1-11/2 hours or until soft through.
When cooked cut the potatoes in half and scrape the insides into a ricer or sieve and pass. Whilst the potato is still warm add the flour, yolks, parmesan, salt and pepper. Bring together to form a soft dough, trying not to overwork the mix.
Bring a large pan of salted water to the boil.
Roll small amounts of the dough into lengths 1.5 to 2cm thick (½ to ¾ inch), then cut to a similar size.
Carefully lift the shaped gnocchi into the boiling water. Allow them to rise to the surface and then cook for a further 20-30 seconds, before removing and placing on a well-oiled tray.
Place the gnocchi in a fridge to cool and firm up for 30 minutes.
When you are ready to eat, heat your oven to about to degrees c or the lowest setting and put in a tray. Heat a large, non-stick frying pan with a little oil and butter in it, and carefully brown your gnocchi on both sides. You may need to do this in batches. Once golden, keep the gnocchi warm on the oven tray.
Put the 50g of butter straight into the hot pan to melt, throw in the sage leaves and garlic and fry until crisp. Remove from the heat, season with salt and pepper and add some lemon juice.
Quickly spoon over your plated, hot gnocchi and grate over some more parmesan.
