Bring a large pot of salted water to a boil and cook the spaghetti according to package directions until al dente; drain and set aside.
Heat the olive oil in a large pot or Dutch oven over medium heat.
Add the finely diced onion and sauté until translucent, about 5–7 minutes.
Add the pressed garlic and cook, stirring, for 30–60 seconds until fragrant.
Stir in the stewed tomatoes, crushed tomatoes, tomato paste, chicken broth, chopped parsley, sugar, dried oregano, dried basil, salt, and black pepper.
Bring the sauce to a gentle simmer, cover, and cook for 20–30 minutes, stirring occasionally; you may simmer up to 2 hours for deeper flavor if desired.
If you prefer a smoother sauce, blend with an immersion blender until you reach the desired texture, or leave it chunky and break larger tomato pieces with a spoon.
Toss the drained spaghetti with the marinara in the pot to coat, or serve the pasta plain and spoon the sauce over individual portions, then serve immediately.
