Step 1

Preheat oven to 375°F. Lightly coat a 9-by-13-inch baking dish with cooking spray and set aside. Melt butter in a medium saucepan over medium-high heat. Add flour and whisk until combined. Gradually whisk in milk and broth; cook, whisking often, until the mixture thickens, 2 to 3 minutes. Add Parmesan and salt; whisk until smooth. Remove from heat and set aside.

Step 2

Heat oil in a large skillet over medium-high heat. Add mushrooms, Italian seasoning and pepper; cook, stirring often, until all liquid has evaporated and the mushrooms are golden brown, 12 to 14 minutes. Remove from heat and set aside.

Step 3

To assemble lasagna, spread 3/4 cup cheese sauce on bottom of the prepared baking dish. Layer on 3 noodles, evenly spacing the noodles crosswise in the dish. (The noodles will not cover the bottom of the dish but will expand while baking.) Spread half the spinach on top of the noodles. Sprinkle with 3/4 cup mozzarella and top with 3/4 cup cheese sauce. Layer on 3 noodles, half each of the mushrooms and chicken, 3/4 cup mozzarella and 3/4 cup cheese sauce. Repeat the layers once, starting with noodles and ending with mushrooms and chicken. Sprinkle with 3/4 cup mozzarella. Add the remaining 3 lasagna noodles; spread the remaining cheese sauce over the noodles and sprinkle with the remaining mozzarella.

Step 4

Lightly coat a piece of foil with cooking spray; cover the dish, coated-side down. Bake for 30 minutes; uncover and bake until golden brown and bubbly, about 15 minutes more. Let the lasagna stand for 15 minutes before serving.