Preheat the oven to 180°C fan (200°C/400°F/Gas Mark 6).
Halve and de-seed the butternut squash. Whack them in the hot oven for 50 minutes or until soft.
Spinach filling time. Finely chop the garlic and throw it in a pan with some olive oil. Cook for 30 seconds, and then drop in the spinach. Once wilted (1 minute), add 6–7 chopped sage leaves. Then, add 125 g (½ cup) of ricotta and 30 g (½ cup) of grated Parmesan. Season with salt and pepper, remove from the heat and set aside.
Take your butternuts out of the oven (remove and discard their skins). Place one of them in a blender. Pulse, adding water, until you have a puree. Cube your other butternut squash into bite-sized pieces.
In a large bowl, place your butternut puree, your butternut cubes, another 125 g (½ cup) of ricotta, 1 large teaspoon of Dijon mustard and some salt and some pepper. Mix it all together.
Assembly time. First, grab a large baking dish. Start with a layer of butternut mixture. Then layer on some lasagne sheets. Then a spinach layer, topped with a good sprinkle of Parmesan. Then more lasagne sheets. Then more butternut squash filling. Then more lasagne sheets. Then, finally, spoon over the remainder of your butternut squash and spinach mixture to make the top layer. Cover with sliced mozzarella and sprinkle the rest of your grated Parmesan over the top.
Add a drizzle of olive oil, a good grinding of black pepper, and put into the hot oven for 45 minutes, or until golden brown on top.
Check the lasagne 15–20 minutes into cooking – if the top is browning too quickly, just put on some foil, and then remove it 5 minutes from the end. Enjoy!