Spicy Sausage Lasagna

Name: Spicy Sausage Lasagna
Ingredients: 2 tablespoons unsalted butter, plus more for buttering the dish; 1/4 cup all-purpose flour; 4 cups milk; 3 garlic cloves, smashed; Pinch freshly grated nutmeg; Kosher salt; 12 lasagna noodles; 2 tablespoons extra-virgin olive oil; 12 ounces hot Italian sausage, casings removed; One 10-ounce box frozen spinach, thawed and squeezed dry; 1/4 teaspoon crushed red pepper flakes; 1 cup lightly packed fresh basil leaves, torn plus more for garnish (optional); 1 cup shredded mozzarella; 1/4 cup freshly grated Parmesan; 3 cups prepared marinara sauce