Bring a large pot of lightly salted water to the boil. Add lasagne and cook for 8 to 10 minutes or until al dente; drain.
In large frying pan over medium heat, saute onion in the olive oil until tender. Add spinach and stir. Remove from heat and stir in the pesto. In a large bowl mix ricotta cheese, egg, salt, pepper and nutmeg.
In a 3 litre greased baking dish, layer the lasagne sheets then the spinach mixture, followed by the ricotta mixture. Sprinkle with mozzarella. Repeat the layers ending with lasagne sheets on top. Spread the cheese sauce over the top and sprinkle with parmesan.
Cover with foil and bake in a preheated oven for 45 to 55 minutes.
Parmesan cheese is not truly vegetarian, as it contains animal rennet. To make this dish 100% vegetarian, omit the cheese or find a suitable vegetarian substitute made without animal rennet. In supermarkets look for the 'parmesan style hard cheeses' which are suitable for vegetarians.