Preheat oven to 350 degrees. Butter a deep dish lasagna pan.

Bring a large pot of salted water to a boil and cook the penne according to the package directions. Reserve about ½ cup of the starchy pasta water and drain the penne.

Melt butter in a large skillet over medium heat. Whisk in the flour. Continue to whisk the flour and butter as it cooks for a minute. Add about 1 cup of the milk and whisk well to dissolve any lumps. Add the rest of the milk, 1 teaspoon of salt, ½ teaspoon pepper, and the nutmeg. Bring the mixture to a boil, whisking constantly. Reduce the temperature and simmer the mixture until thick, about 15 minutes. Ladle about ¾ cup of the béchamel into the bottom of the prepared pan. Stir the penne into the rest of the sauce. If the sauce is too thick, stir in some of your reserved pasta water.

Layer half the pasta in the pan, spread half the pesto on top and sprinkle on ½ cup of Parmesan cheese. Repeat one more time. Sprinkle a little bit of salt and pepper on top and bake for 35 – 40 minutes or until the pasta is bubbly and the top is beginning to brown.