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Heat the oven to 220C/fan 200C/gas 7. Toss the squash together with the olive oil in a roasting tin. Roast the squash for 30 mins until soft and golden. While the squash is roasting, mix the crème fraîche with half the Parmesan, then set aside.
When squash is ready, leave to cool slightly, then peel the skin away from the flesh. In a separate bowl, beat the ricotta with the chopped sage and remaining Parmesan and fold through the squash with some seasoning, trying not to break it up too much.
Assemble the bake by spreading a little of the crème fraîche mix over a gratin dish. Then lay some of the lasagne sheets over and splodge some of the ricotta and squash mix over the pasta with more crème fraîche. Use all the ricotta mix to fill the bake, but make sure you keep some crème fraîche for the top. Finally, spread the crème fraîche over the top layer of lasagne, then scatter with the whole sage leaves. The bake can now be frozen; defrost thoroughly before cooking. Bake for 25 mins until bubbling and golden, then serve cut into squares.