Bring a pot of water to a boil. Cook the spinach for 30 seconds. Drain well and chop. Set aside.
In a large skillet, heat a drizzle of olive oil over medium heat. Add the salmon and cook for 4 to 5 minutes per side. Remove from heat and flake into pieces. Set aside. (The salmon does not need to be fully cooked, as it will cook again in the lasagna.)
In the same skillet, add another drizzle of olive oil. Sauté the onion for 1 to 2 minutes. Add the leeks, oregano, and garlic, season with salt and pepper, and cook for 4 to 5 minutes, until the vegetables are tender. Set aside.
In a saucepan, heat the olive oil over medium heat. Add the flour and cook for 1 minute, stirring constantly. Gradually pour in the milk, stirring to avoid lumps.
Add the Dijon mustard and cook until thickened. Season with salt and pepper to taste.
In a bowl, combine the ricotta, spinach, egg, and a little oregano. Season lightly with salt and pepper.
Spread a thin layer of béchamel sauce on the bottom of a large ovenproof dish. Place 2 lasagna sheets on top. Add another layer of béchamel sauce, then the leek mixture, salmon, and a little mozzarella cheese. Add 2 more lasagna sheets, then spread the ricotta mixture over the top. Add two more lasagna sheets, then repeat with the béchamel sauce, mushrooms, salmon, and mozzarella. Finish with two more lasagna sheets, the remaining béchamel sauce, and the remaining mozzarella.
Cover with aluminum foil and bake for 30 minutes. Remove the foil and continue baking for 10 to 15 minutes, until the top is golden brown.
