Make the marinara sauce: Heat the olive oil in a large pot over medium heat. Add the sliced garlic and cook until fragrant and just beginning to soften. Stir in the tomato puree, diced tomatoes, Italian seasoning, dried basil, red pepper flakes, salt, and black pepper, mixing well to combine. Reduce the heat to low and let the sauce simmer for 35 to 45 minutes, stirring occasionally. Taste the sauce and add sugar if needed to balance the acidity. For a richer finish, stir in butter at the end of cooking if desired.

Make the filling: In a mixing bowl, combine the ricotta cheese, egg, Parmesan cheese, and Italian cheese blend. Stir until the mixture is smooth and fully combined.

Cook the manicotti according to the package directions until al dente. Drain and lay the pasta out on wax paper so the pieces do not stick together.

Preheat the oven to 350°F. Spread a generous layer of marinara sauce across the bottom of a 13x9 inch baking dish.

Carefully fill each manicotti shell with the cheese mixture using a spoon or butter knife. Arrange the filled manicotti in a single layer in the baking dish. Cover the pasta completely with the remaining marinara sauce, then sprinkle the mozzarella cheese evenly over the top. Add a light sprinkle of Parmesan cheese if desired.

Cover the baking dish with foil and bake for 30 to 35 minutes. Remove the foil and continue baking for an additional 10 to 15 minutes, or until the cheese is melted and slightly golden. Let the manicotti rest for 10 to 15 minutes before serving so it sets properly. Garnish with finely chopped parsley.