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 minutes.
Add the pressed garlic and cook, stirring, for 30 to 60 seconds until fragrant.
Stir in the stewed tomatoes, crushed tomatoes, tomato paste, chicken broth, chopped parsley, sugar, oregano, basil, salt, and black pepper until combined.
Bring the sauce to a simmer, cover, and cook over low heat for 20 to 30 minutes, stirring occasionally; simmer longer (up to 2 hours) if desired for deeper flavor.
Use an immersion blender to puree the sauce to your preferred texture, or leave it chunky by mashing larger pieces with a wooden spoon.
Meanwhile, cook the spaghetti according to package directions until al dente, then drain in a colander.
Add the drained pasta to the sauce and toss to combine, or serve the pasta plain and spoon the marinara over individual portions.