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, and chicken broth.
Add the chopped parsley, sugar, oregano, basil, salt, and black pepper; stir to combine.
Bring the sauce to a simmer, cover, and cook for 20–30 minutes, stirring occasionally; simmer longer if desired for deeper flavor.
If you prefer a smoother sauce, use an immersion blender to blend until you reach the desired texture; otherwise break up larger tomato pieces with a wooden spoon.
Meanwhile, cook the spaghetti according to package directions until al dente, then drain in a colander.
Return the drained pasta to the pot with the sauce and toss to coat, or serve the pasta plain and ladle the marinara over individual portions.