Mix the potatoes and oil, spread onto a baking tray lined with baking paper.
Bake for approx. 25 mins. in the centre of an oven preheated to 200°C. Remove from the oven, push through a sieve into a bowl and leave to cool a little.
Add the flour, egg and salt, mix by hand. The mixture should still be moist but no longer stick to your hands. Add a little more flour if necessary. On a lightly floured surface, shape the dough into 4 rolls (each approx. 1 cm in diameter). Cut the rolls into approx. 2 cm long pieces, place the gnocchi on a floured towel.
Cook the gnocchi in batches in gently simmering salted water for approx. 5 mins. until they float to the surface. Remove with a slotted spoon, drain, cover and keep warm.
Heat the clarified butter, sauté the onion, add the feta, water and oil, puree. Stir in the rosemary and chilli pepper, heat gently. Serve the gnocchi with the feta cream sauce, sprinkle the walnuts over the top.