Melt the butter in a large saucepan over medium heat. Add the mushrooms and cook, stirring often until the mushrooms are browned and most of the liquid has evaporated, about 5 to 7 minutes. Add the onions and garlic to the pan and saute until soft and translucent, 3 to 4 minutes. Add the flour and cook, stirring with a wooden spoon, to make a light roux , about 2 minutes. Whisking constantly, slowly add the milk and continue to cook, stirring occasionally until thickened, 5 minutes. Add 1 1/2 teaspoons of the salt, pepper, nutmeg, spinach and 1 1/2 cups of the Parmesan and cook, stirring, until thickened, about 2 minutes. Remove from the heat. Place a piece of plastic wrap directly on the surface of the bechamel sauce until ready to assemble the lasagna . Set a large, 12-inch saute pan over medium heat and add the olive oil. Season the chicken with the Essence and remaining 1/2 teaspoon of kosher salt and place in the hot pan. Sear the chicken, stirring occasionally, until golden brown and cooked through, about 4 to 5 minutes. Transfer to a plate to cool and set aside. When cool, cut into bite size pieces. Preheat the oven to 375 degrees F. Coat a 9 by 13 by 3-inch casserole with olive oil, and spread about 1/2 cup of the bechamel sauce on the bottom of the dish. Lay 3 sheets of pasta across the bottom of the dish and spread 3/4 cup of the bechamel sauce over the pasta. Sprinkle 1/4 of the chicken over the bechamel sauce, then sprinkle with 1/4 cup of the remaining Parmesan. Lay another 3 sheets of pasta over the chicken . Repeat 2 additional times with the remaining bechamel sauce, chicken, Parmesan, and pasta, ending with a layer of pasta covered with bechamel sauce. Sprinkle the remaining 1/2 cup of Parmesan over the bechamel sauce and scatter the butter pieces over the top. Place the casserole on a parchment paper-lined baking sheet and bake, uncovered, until bubbly and well browned, about 45 minutes. Remove from the oven and allow to cool for at least 20 minutes before serving. Note: You may need slightly less than the entire package of lasagna noodles , depending on the pan used for the casserole. Also, you may be able to fit more than 3 pieces of pasta in each layer, depending. The pasta can be broken into smaller pieces to fill in the gaps. Combine all ingredients thoroughly. Yield: 2/3 cup Recipe from "New New Orleans Cooking", by Emeril Lagasse and Jessie Tirsch. Published by William Morrow, 1993.
