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.
Combine all ingredients thoroughly.
Recipe from "New New Orleans Cooking", by Emeril Lagasse and Jessie Tirsch. Published by William Morrow, 1993.
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.
Recipe from Emeril Lagasse