Preheat the oven to 180C/350F/Gas 4.
Heat the olive oil in a large heavy-based pan. Add the pumpkin and cook, covered, stirring gently from time to time, until the pumpkin is tender, up to one hour.
Season the pumpkin with plenty of paprika, a grating of nutmeg, freshly ground black pepper and a little salt, to taste.
Add the ricotta cheese while it's still on the heat. Stir well then remove from the heat. Cover and allow mixture to stand.
For the béchamel sauce, melt the butter in a small saucepan without letting it brown. Add the flour little by little, stirring constantly to prevent lumps from forming.
When the flour has evenly absorbed the butter, continue to cook, stirring, for 4-5 minutes.
Add the 350ml/10½fl oz milk a little at a time, strirring constantly to make a smooth, thick and creamy sauce. Season.
Arrange a layer of lasagne pasta sheets in the bottom of a large ovenproof dish. Spoon over a third of the pumpkin mixture. Lay more pasta sheets over the top, followed by another layer of pumpkin and another layer of pasta, followed by a final layer of the pumpkin mixture. Finish with a layer of lasagne pasta sheets.
Pour the 150ml/5¼fl oz milk over the top and allow it to drain through to all pasta layers.
Bake for ten minutes in the preheated oven, remove and spread the béchamel sauce on top.
Bake for a further 15-20 minutes or until all the milk has been absorbed by the pasta and the lasagne looks cooked and is beginning to go crisp.
Remove from the oven, sprinkle with parmesan and place back in the oven for a few minutes - take care not to burn the pasta or the béchamel on top. Serve immediately.