Melt 50g of the butter in a pan and whisk in the flour. Cook for 1 to 2 minutes, then whisk in the milk till smooth. Season with sea salt and freshly ground black pepper, add the bay leaf and simmer for 5 minutes. Turn off the heat.
Remove the stalks from the spinach, then wilt with the remaining 20g butter in a covered pan. When wilted, drain, then, when cool enough to handle, squeeze out the liquid.
Chop the spinach and mix with the ricotta, a ladleful of the white sauce and a good grating of nutmeg. Season.
In a baking dish, layer the lasagne sheets, white sauce, spinach mixture and a grating of Parmesan. Finish with a layer of pasta topped with sauce and more Parmesan.