If you've no tinned salmon in the larder, then tins of tuna will work well here too.
First make the béchamel sauce. Make a roux by melting the butter over a low heat in a heavy based saucepan. Sprinkle the flour into the butter and stir with a balloon whisk. Cook gently for 2 to 3 minutes, until the roux is an off-white colour. Don’t overcook the flour or it will make the sauce discoloured.
Slowly pour the milk onto the roux and increase the heat
to medium. Whisk continuously until it comes to the boil and add some salt and white pepper. Cook for a further 10 minutes to ensure the flour is cooked through.
In the meantime, melt the 50g of butter in a pan over a
medium heat. Add the spinach and shallots and season with a little salt and pepper. Once the spinach has wilted, remove the pan from the heat.
Preheat the oven to 180°C/350°F/gas mark 4. Lightly oil the bottom of a 23cm x 33cm baking dish.
Flake the salmon into generous bite-sized pieces with a fork or your fingers. Cover the base of the dish with one-third of the lasagne sheets, then add one-third of the
salmon, one-third of the spinach and shallots, one-third of the béchamel sauce and one-third of the cheese. Repeat this process twice more, finishing with the cheese on top.
Bake in the oven for about 30 minutes, until the lasagne is hot and bubbling and the cheese has melted. Allow
the lasagne to cool slightly and firm up before slicing