Bechamel sauce.
Heat milk in a medium saucepan until hot. Melt butter in a large saucepan over medium-low heat; whisk flour in until blended. Cook 2 minutes. Gradually begin adding hot milk, whisking constantly, until incorporated and sauce is smooth. Stir in salt, pepper, nutmeg and bay leaf. Raise heat to medium and bring to a simmer. Simmer 3 minutes longer. Pour into a container, cover surface directly with plastic wrap, and cool. Can be made up to three days ahead and refrigerated.
Tomato sauce.
Heat oil in a large sucepan over medium-high heat. Add sausage and brown 3 minutes, breaking up clumps with a wooden spoon. Add onion; cook 6 minutes, stirring frequently, until lightly browned. Pour in marinara sauce; bring to a boil, reduce heat to low, partially cover pot, and simmer for 15 minutes. Remove from heat and cool. Can be made up to three days ahead and refrigerated.
Lasagna.
Evenly spread 1 cup of the tomato sauce over the bottom of a 13x9x2 baking pan. Arrange layer of noodles over sauce. Spread noodles with 1/3 bechamel sauce. Spread 1/2 ricotta and 1/2 mozarella and 1/2 the remaining tomato sauce. Sprinkle 1/4 cup parmesan. Make a second layer of noodles. Spread 1/2 of the remaining bechamel sauce then the remaining ricotta, mozarella and tomato sauce and another 1/4 cup parmesan. Top this with a final layer of noodles. Spread the remaining bechamel sauce, sprinkle with the remaining parmesan and sprinkle with pepper.
Heat oven to 375 F. Bake 1 hour or until lasagna is bubbling and top is lightly browned. Let stand uncovered 20 minutes before cutting and serving.