For the Béchamel sauce: heat butter in a medium saucepan until melted. Add flour and stir until smooth. Cook over medium heat until light golden brown, about 6 to 7 minutes
Meanwhile, heat milk in a separate pan until it is about to boil. Add milk to butter mixture 1 cup at a time, whisking continuously until smooth and bring to a boil. Cook 30 seconds and remove from heat. Season with nutmeg and salt then set aside
Preheat oven to 425° F
Bring a large pot of water to a boil and cook pasta half the time indicated on the package directions
Drain pasta and return to pan. Add Marinara sauce, Béchamel sauce, mozzarella, and Parmigiano cheese to the pasta and mix well
Divide pasta and sauce mixture among six individual au gratin dishes. Sprinkle with panko breadcrumbs
Bake for about 20 minutes, until bubbling and crusty on top. Serve immediately
