Preheat the oven to 200°C/gas mark 6. Heat the oil in a large frying pan and cook the diced onions for 5 minutes until softened. Add the pork mince and fry for a further 5 minutes, until browned all over. Stir in the mushrooms and cook for 3-4 minutes or until softened.
Pour ¾ of the tomato and mascarpone sauce into the frying pan, followed by the tin of chopped tomatoes and sage. Simmer for 5-6 minutes, until slightly thickened. Add the spinach and cook until wilted. Mix well to ensure it is covered with sauce.
Begin to layer the lasagne in an ovenproof baking dish. Start with a layer of the pork mixture then cover with a layer of butternut squash lasagne sheets. Repeat this until the mix is used up, ensuring the butternut squash is the final layer.
Top with the remaining tomato and mascarpone sauce and a generous topping of parmesan cheese. Bake in the oven for 45-50 minutes until a golden brown crust has formed. Take out of the oven and allow to sit loosely covered with foil for 15 minutes. Serve alongside the salad leaves.