Bring the milk to a boil in a pot over medium heat. Add the salmon and cook for 8 to 10 minutes over medium-low heat. Transfer the salmon onto a plate and cut it into pieces. Set the milk aside.
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 dill 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, salmon pieces, white sauce, spinach mixture, cheddar and a grating of Parmesan. Finish with a layer of pasta topped with sauce and more cheddar and Parmesan.
