Preheat the oven to 180°C (gas 4). Mix together the ricotta, Parmesan and egg in a large bowl.
Spread 250ml of the pasta sauce over the base of a lightly oiled, large baking dish, measuring about 23 x 33cm and about 7cm deep.
Arrange three of the lasagne sheets side by side in the dish. Spread with a quarter of the ricotta mixture then top with a third of the spinach. Repeat layering twice more with the sauce, lasagne, ricotta mixture and spinach. Top with remaining three sheets of lasagne.
Spread the remaining ricotta mixture and sauce over the top, then gently press the lasagne sheets down into the dish, so the sauce comes up around the sides. Cover the dish with foil.
Bake for 35 minutes, then uncover, sprinkle with the mozzarella and bake for a further 20 minutes until golden and bubbling.
Allow to stand for 10 minutes before cutting into rectangles to serve warm.
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.
You can make this lasagne with other ingredients. Try frozen, chopped broccoli and some diced, cooked ham; or drained, canned tuna in place of the spinach layers.