First make the white sauce. Melt the butter and flour together in a big saucepan, and cook the paste for a couple of mins. Add the bay leaves, then gradually whisk in the milk, little by little, until you have a nice smooth sauce. Bubble for a few mins, then season with salt, pepper and nutmeg. Remove the bay leaf, cover with cling film (lay it directly onto the sauce) until ready to use.
Gently cook the onions in the oil until really soft. Put the kettle on. Put half the spinach in a big colander and pour over the boiling water to wilt. Repeat with the rest of the spinach, let it cool a little then squeeze as much liquid as you can out of the spinach. Roughly chop and stir into the onions with the ricotta, a small ladle of the white sauce and most of the parmesan. Season well.
Heat oven to 200C/180C fan/gas 6. Spread a quarter of the sauce into the base of a big baking dish, then dollop over a third of the spinach mixture. Top with a layer of lasagne sheets, then repeat the layers – a quarter of the sauce, third of the spinach mix and some lasagne sheets. Finish with the remaining sauce and scatter over the remaining parmesan. Bake for 40-50 mins until the lasagne is cooked through and the top golden, scattering over the pine nuts with 10 mins to go.