Pass the potatoes through a food mill into a bowl, allow to cool slightly. Add the flour and cheese, mix together. Add the ricotta, egg yolks and marjoram, season, 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 potato mixture into 4 rolls (each approx. 2 cm in diameter). Cut the rolls diagonally into approx. 1 cm pieces. 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.
Prawns & vegetables
Heat the oil in a non-stick frying pan. Fry the prawns for approx. 1 min. on each side, remove from the pan. Cut half of the prawns into approx. 1 cm pieces. Heat a dash of oil in the same pan, stir fry the courgette for approx. 3 mins. Add the tomatoes and prawns, cook briefly, season. Pour in the stock, add the gnocchi and heat through.