1 For the Béchamel sauce: In a saucepan, melt the butter over medium heat then whisk in the flour.
2 Cook the “roux” for 2 to 3 minutes then, off the heat, add the milk a little at a time, whisking constantly.
3 Add the pan back to the heat and cook for a few minutes to thicken then add the Boursin. Set aside.
4 Cut the salmon fillet into 4 steaks then brown them for 2 minutes each side in a frying pan with a tablespoon of olive oil.
5 Remove the salmon and add the spinach to the frying pan and stir for 30 seconds until cooked.
6 To assemble the croque monsieur: Take 4 slices of wholemeal sandwich bread, spread with a layer of Béchamel then half the spinach, add a slice of salmon then finish with the remaining spinach. Cover the sandwiches with the remaining slices of bread.
7 Heat the frying pan to 2/3 of the maximum power of your stove with a tablespoon of oil and brown the croque monsieur for 1 to 2 minutes on each side.
Tip: You can substitute smoked salmon for the fresh salmon.