How to Make It
Preheat the oven to 425°. In a small saucepan, melt the butter over medium heat. Add the flour and whisk for 3 minutes, or until it becomes light golden brown in color. Slowly whisk in the milk and cook till it thickens. Remove the pan from the heat and whisk in the Dijon mustard, soy sauce and nutmeg. Season generously with sea salt and freshly ground pepper. Whisk ¾ cup of water into the white sauce.
In a 9-by-11-inch ovenproof baking dish, spread 1/2 cup of the white sauce and top with 4 of the lasagna noodles. Add a layer of chicken and spinach, then top with some of the white sauce. Repeat with the remaining noodles, chicken, spinach and white sauce, to make three layers of lasagna, ending with white sauce on top. Sprinkle with the Parmigiano-Reggiano cheese and dot with the mozzarella. Cover the dish with aluminum foil.
Bake in the preheated oven for 30 minutes, then uncover the baking dish and bake for 10 minutes, until the noodles are cooked. Allow the lasagna to rest for 10 minutes before serving.