Bring a large pot of salted water to a boil. Cook the spinach for 2-3 min. Stirring partway through to make sure it evenly cooks. Drain in a fine-mesh sieve. Once it is cool squeeze it out with your hands, getting out as much as possible. Finely chop and set aside.

Meanwhile, in a large skillet over medium-high heat melt the 1 Tbsp.butter and add the shallots and garlic, stir for 1 min. Add the mushrooms and a 1/4 tsp. salt and pepper. Cook, stirring until the mushrooms have let off their liquid, about 5 min. Remove from heat and set aside to cool.

In a large saucepan melt the remaining 4 Tbsp. butter over medium heat. Add the flour and let cook for 2 min. stirring with a whisk. Slowly add in the milk whisking constantly. Let the mixture come up to a simmer until it is thickened, whisking often. Add 1/2 tsp. salt, 1/4 tsp. black pepper, nutmeg, and 1 cup of Parmesan now stirring with a wooden spoon for about 2 min. until thickened. Remove from heat.

In a medium bowl combine the ricotta and the mozzarella with a 1/4 cup of the sauce.

Preheat oven to 350F. Spray a 9x13" pan with cooking spray. Spoon 1/2 cup of the sauce into the bottom of the pan. Divide the mushrooms and spinach into four equal parts and sprinkle 1/4 of the mushrooms and spinach over the sauce. Next, arrange a layer of 4 noodles side by side. Spread a layer of the sauce over the noodles now using a bit more than 1/2 cup. Top again with 1/4 mushrooms and spinach and a 1/3 of the ricotta mixture and 6 sliced provolone. Repeat these layers twice more ending with the noodles. Use the rest of the sauce on top of the lasagna noodles and sprinkle with the remaining 1 1/2 cups of Parmesan.

Cover tightly with aluminum foil (you can refrigerate the lasagna now if needed). Bake in the oven for 30 min. Remove the foil and bake for another 10-15 min. until golden brown and bubbling. You may want to run it under a broiler for a minute.

Let rest for 10-15 min. before serving. Serve hot!!