In a large bowl, combine the ricotta cheese, cooked spinach, Parmesan cheese, egg, garlic powder, salt, and black pepper. Mix thoroughly until the filling is smooth and evenly combined, then set aside while preparing the sauce.

Heat the olive oil in a skillet over medium heat and cook the onion for 4–5 minutes until softened. Add the garlic and cook briefly before stirring in the tomato passata, Italian seasoning, salt, and pepper, then simmer for 10 minutes.

Transfer the spinach and ricotta mixture into a piping bag or spoon it carefully into each cannelloni tube. Fill the tubes completely but avoid overpacking them to prevent splitting during baking.

Spread a thin layer of tomato sauce across the bottom of a baking dish and arrange the filled cannelloni in a single layer. Pour the remaining sauce evenly over the pasta, ensuring every tube is well covered.

Sprinkle the mozzarella and Parmesan cheese evenly over the top of the cannelloni. The cheese will melt and form a golden, bubbly crust as it bakes.

Bake at 375°F (190°C) for 35–40 minutes until the pasta is tender and the cheese is golden brown. Let the cannelloni rest for 5 minutes before serving to allow the filling and sauce to settle.