Prepare the Meat Sauce: Heat olive oil in a large skillet or Dutch oven over medium heat until shimmering. Add the finely chopped onion and sauté until soft and translucent, approximately 5–7 minutes. Stir in the minced garlic and cook briefly until fragrant, being careful not to burn it. Add the ground beef and pork, cooking until browned and crumbly. Drain excess fat before incorporating oregano, basil, and red pepper flakes. Stir in the passata and beef broth, then season with salt and pepper. Allow the sauce to simmer gently on low heat for 30–45 minutes, stirring occasionally until thickened and flavorful.
Prepare the Ricotta Filling: In a medium mixing bowl, combine ricotta cheese, Parmesan, and the lightly beaten egg. Add nutmeg, salt, and pepper. Mix until the filling is creamy and uniform, then set aside for later use.
Prepare the Béchamel Sauce: In a medium saucepan, melt the butter over medium heat. Whisk in the flour to form a smooth paste (roux), cooking for 1–2 minutes without letting it brown. Gradually add milk in small increments, whisking continuously to prevent lumps. Cook until the sauce thickens and begins to simmer, then season with nutmeg, salt, and pepper. Remove from heat and set aside.
Assemble the Cannelloni: Preheat the oven to 350°F (180°C). Lightly grease a 9×13-inch (23×33 cm) baking dish. Spread a thin layer of the meat sauce across the bottom. Using a piping bag or small spoon, fill each cannelloni tube with the ricotta mixture and arrange them neatly in the baking dish.
Layer and Bake: Cover the filled cannelloni with the remaining meat sauce, then pour the béchamel evenly over the top. Sprinkle with grated mozzarella and additional Parmesan for a golden crust. Cover the dish with foil and bake for 30 minutes. Remove the foil and continue baking for an additional 15–20 minutes, or until the top is golden and bubbling.
Rest and Serve: Remove from the oven and allow the cannelloni to rest for 10–15 minutes before serving. Garnish with fresh basil leaves for an elegant presentation.