Preheat oven to 350 degrees F. Lightly butter a 9-by 13-inch baking dish.
Melt the butter in medium skillet over medium heat. Add the flour and stir with a wooden spoon until lightly toasted, about 2 minutes. Whisk in the milk and bring to a boil while stirring. Add the garlic, reduce the heat to maintain a simmer and continue to cook, whisking occasionally until thick, about 20 minutes. Stir in the nutmeg and 1 teaspoon salt.
Bring a large pot of salted water to a boil. Add the noodles and cook according to the package directions for al dente. Drain and lay out on a cutting board so they don't stick to each other.
Heat the oil in a large skillet over medium heat. Add the sausage and cook, breaking up the meat with a wooden spoon until browned and cooked through, about 5 minutes. Add the spinach, pepper flakes and 1/2 teaspoon salt and cook, stirring occasionally, until the spinach is dry. Remove from the heat and stir in the basil.
Toss the mozzarella and Parmesan together in a small bowl. Remove the garlic from the cream sauce. Cover the bottom of the prepared dish with 3 noodles. Top with 1/4 cup of the cheese mixture, 3/4 cup marinara, 1/2 cup cream sauce and 1/3 of the sausage mixture. Repeat the layers until all the noodles have been used. Top the final sausage layer with the remaining cream sauce, marinara and cheese, making sure the edges of the noodles are covered. Bake until bubbly, about 45 minutes. Let lasagna stand for 10 minutes before slicing. Garnish with more basil if using.
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