Make the mushroom ragu. Mushroom ragu will take about an hour to prepare but can be made up to 5 days in advance.

Add the butter to a 3 quart or larger saucepan and set it over medium heat. When the butter is melted, add the flour, stirring constantly with a wire whisk. Cook, whisking constantly, for 2 minutes.

Add the half and half gradually, whisking constantly as you slowly pour it into the saucepan. Add the nutmeg and red pepper flakes and bring the sauce to a simmer.

Cook over medium-low heat, whisking frequently, until the béchamel is thickened and does not taste like raw flour, about 12 minutes. (The sauce should be thick enough to coat a spoon; see photos above.)

Remove the saucepan from the heat and whisk in 4.5 oz grated parmesan, stirring until the cheese is melted.

Stir the fresh parsley and the mushroom ragù into the béchamel. Add salt and pepper to taste, then cover and set aside. (*See notes below for make-ahead instructions.)

Heat the oven to 350 degrees F (176 degrees C). If using oven ready no-boil lasagna noodles, see the note below about pre-soaking them before layering them into the lasagna. If using fresh pasta, roll the pasta dough into thin sheets and then cut the sheets to fit the pan.

Layer #1: Spread a thin layer of the béchamel and mushroom mixture over the bottom of the dish. Lay ⅓ of the noodles in an even layer over the sauce. Spread ⅓ of the béchamel and mushroom mixture over the noodles and top with ⅓ of the cheese.

Layer #2: Add another layer of noodles. Spread ⅓ of the béchamel and mushroom mixture over the noodles and top with ⅓ of the cheese.

Layer #3: Lay the remaining noodles over the last layer, spread with the remaining béchamel and mushroom mixture, and top with the remaining cheese.

Cover the pan with foil and bake for 30 minutes. Remove the foil and bake for another 5-10 minutes, until the cheese across the top of the lasagna is melted and the sauce is bubbling around the edges of the pan.

Remove the lasagna from the oven and let stand for 5-10 minutes before slicing and serving. Sprinkle with more fresh parsley before serving.