Mix the first 5 ingredients, bring to a boil and poach the salmon for 5 minutes.
In the meantime, prepare a bechamel sauce by melting the butter and adding the flour. Then, remove from the heat and pour the milk while stirring. Reheat while whipping until the milk reaches a boil.
Incorporate the spinach into the sauce.
Put the salmon on a slab and brush with the béchamel sauce. Sprinkle with Mozzarella cheese, and put in the oven until desired doneness.
