Bring a large pot of salted water to a boil. Add spaghetti and cook according to package instructions until al dente, about 9-11 minutes. Drain and set aside.
While the pasta cooks, heat olive oil in a large skillet over medium heat. Add chopped onion and sauté until translucent, about 5 minutes.
Add minced garlic to the skillet and cook for 1 minute until fragrant, stirring frequently to prevent burning.
Pour in the crushed tomatoes, then add dried oregano, dried basil, salt, black pepper, red pepper flakes, and sugar. Stir to combine and bring the sauce to a simmer.
Reduce heat to low and let the sauce simmer gently for 15 minutes, stirring occasionally.
Stir fresh parsley into the sauce and remove from heat.
Preheat the oven to 375°F (190°C).
Slice the Italian bread loaf in half lengthwise.
In a small bowl, mix softened butter, minced garlic, chopped parsley, and salt until well combined.
Spread the garlic butter mixture evenly over the cut sides of the bread.
Place the bread halves on a baking sheet, cut side up, and bake in the preheated oven for 10-12 minutes until the edges are golden and crispy.
Toss the cooked spaghetti with the tomato sauce until well coated.
Serve the pasta topped with grated Parmesan cheese alongside slices of warm garlic bread.