Bring a large pot of lightly salted water to the boil. Add lasagne sheets and cook for 8 to 10 minutes or until al dente; drain.
In a frying pan over medium-high heat, cook mushrooms, onions and garlic in olive oil until onions are tender. Drain excess liquid and cool. Boil spinach for 5 minutes. Drain, then squeeze out excess liquid. Chop spinach.
Combine 750g ricotta cheese, 80g of Pecorino cheese, cooked spinach, salt, oregano, basil, pepper and egg in a bowl. Add cooled mushroom mixture. Beat with an electric mixer on low speed for 1 minute. Lay 5 lasagne sheets in bottom of a large baking dish. Spread one third of the cheese/spinach mixture over lasagne sheets. Sprinkle some of the grated mozzarella and Parmesan cheese on top. Spread 1/3 of the passata over cheese. Repeat layering 2 times.
Cover dish with foil and bake in a preheated oven for 1 hour. Cool 15 minutes before serving.