For the Homemade Marinara Sauce:

1. In a large saucepan, heat the olive oil over medium heat. Add the chopped onion and sauté for about 3-4 minutes until it becomes translucent.
2. Add the minced garlic and sauté for another 30 seconds, until fragrant.
3. Pour in the crushed tomatoes, sugar (if using), dried oregano, dried basil, and red pepper flakes. Stir well to combine.
4. Season the sauce liberally with salt and black pepper. Reduce the heat to low and let the sauce simmer for about 20-30 minutes, stirring occasionally. The sauce will thicken, and the flavors will meld together beautifully.
5. Once the sauce has reached your desired consistency and flavor, remove it from the heat and set aside. If you prefer a smoother sauce, you can use an immersion blender to puree it.

For the Manicotti Filling:

1. Cook the manicotti pasta according to the package instructions minus 1-2 minutes. Drain and rinse with cold water to stop the cooking process. Set aside.
2. In a large mixing bowl, combine the well-drained thawed spinach, ricotta cheese, shredded mozzarella, grated Parmesan, minced garlic, egg, dried oregano, dried basil, salt, and black pepper. Mix until all the ingredients are well combined.

Assembling the Manicotti:

1. Preheat your oven to 375°F (190°C).
2. Take a baking dish and spread a thin layer of the homemade marinara sauce evenly on the bottom.
3. Carefully fill each cooked manicotti tube with the ricotta and spinach mixture using a spoon or a piping bag. Arrange the filled manicotti in the baking dish.
4. Pour the remaining marinara sauce over the top of the manicotti, making sure they are well coated.
5. Sprinkle additional mozzarella and Parmesan cheese on top. The more the better!
6. Cover the baking dish with aluminum foil and bake for 20 minutes. Then, remove the foil and bake for an additional 10-15 minutes until the cheese is bubbly and slightly golden.
7. Once baked, let it cool for a few minutes, garnish with fresh basil, and serve hot.