Meanwhile, cook pasta shells in boiling salted water to packet instructions (minus a minute) until al dente. Drain, rinse under cold water to stop the cooking process so they don’t stick together, and set aside. In a large bowl combine the beaten eggs, ricotta, yoghurt, half the grated cheese and nutmeg, lemon zest, spinach and parsley; beat until smooth and season to taste. Using a dessert spoon, fill the pasta shells with ricotta mixture, placing each shell on the butternut base. Scatter with remaining grated cheese and rasp of nutmeg, cover with foil and return to the oven for 20 minutes. Remove foil and cook for another 20 minutes until bubbling and golden. Leave to cool a little before serving.