Heat up the oven broiler.
Bring a large pot of water to a boil.
Clean the mushrooms: gently remove the stems and scrape out the underside gills. Spray both sides with cooking spray and broil, 5 minutes per side a couple inches under the heat. Remove from the oven and set aside to cool. Once cool enough to handle, chop into half-inch pieces.
Add the pasta to the boiling water and cook al dente according to package directions.
While pasta is cooking, heat a medium saucepan over medium-high heat. Add the butter and flour and stir until smooth. Cook a minute to make a roux. Slowly whisk in the milk until smooth. Remove from the heat and whisk in the Dijon mustard, 1 teaspoon kosher salt, and a pinch of fresh cracked pepper. Add two-thirds of the cheese (reserving the rest). Add the spinach and chopped mushrooms.
Once the pasta is done, add to the prepared sauce. Transfer all to a bake dish (I used a 11x8 pyrex). Top with the remaining cheese.
In a small bowl, combine the parmesan cheese, panko breadcrumbs, and olive oil. Sprinkle over the casserole. Bake at 350F for 20 minutes or until bubbly. Yields: 6 servings.