Preheat the oven to 180°C. Lightly oil a large ovenproof baking dish measuring about 23×33 cm and about 7 cm deep.
Mix together the ricotta, parmesan and egg in a large bowl. Spread 1 cup of the pasta sauce over the base of the oiled baking dish. Arrange four of the lasagne sheets to cover the base of the dish; if necessary, overlap them a little. Spread with a quarter of the ricotta mixture, then top with a third of the spinach. Repeat layering twice more with the sauce, lasagne sheets, ricotta mixture and spinach. Top with the remaining four sheets of lasagne.
Spread the remaining ricotta mixture and pasta sauce over the top, then gently press the lasagne sheets down into the dish, so the sauce comes up around the sides. Use a sheet of foil to cover the dish.
Bake for 35 minutes, then uncover, sprinkle with the mozzarella and bake for a further 20 minutes until the cheese is golden and bubbling. Remove from the oven and allow to stand for 10 minutes before cutting into rectangles to serve.