Bring a large pot of lightly salted water to the boil. Add the lasagne sheets and olive oil; cook until al dente, 8 to 10 minutes; drain.
Cook the minced beef and sausagemeat in a large pot over medium heat; drain. Stir in the mushrooms, garlic salt, oregano, thyme, basil and passata. Season with salt and pepper; simmer 30 minutes.
Meanwhile, mix together the ricotta cheese, eggs and Parmesan cheese in a bowl.
Ladle enough of the meat sauce into a 23x33cm or similar size baking dish to cover the bottom in a thin layer. Lay three lasagne sheets on top of the meat sauce. Spread about 1/4 of the ricotta cheese mixture over the lasagne sheets. Sprinkle about 1/3 of the mozzarella cheese over the ricotta cheese mixture and then ladle about 1/3 of the meat sauce over the mozzarella cheese. Repeat layering twice more, topping with the remaining mozzarella cheese.
Bake in preheated oven 90 minutes. Allow to sit 10 to 15 minutes before serving.