Heat a Dutch oven or large 5–7 quart pot over medium-high heat and add 1 tablespoon olive oil.
Add the ground turkey, spread it out, season with 1/2 teaspoon salt and 1/4 teaspoon pepper, and cook, breaking it up, until browned and cooked through, about 7–10 minutes.
Add the minced garlic and diced onion and cook with the meat about 1 minute until fragrant and softened.
Pour in 3 cups marinara sauce, 2 1/2 cups low-sodium chicken broth, 1 teaspoon balsamic vinegar, 1 tablespoon Italian seasoning, and 1 tablespoon chopped fresh parsley; stir gently to combine.
Break the 8 ounces spaghetti in half and add to the pot in small handfuls, spreading and alternating directions so the pasta is submerged and less likely to clump; gently push under the liquid but avoid stirring immediately.
Bring to a boil, then cover and reduce heat to medium to maintain a simmer; cook covered for 13–15 minutes or until the pasta is tender, stirring every 5 minutes to prevent clumping.
When the pasta is tender, remove from heat and let stand 2–3 minutes to absorb sauce, then serve garnished with additional parsley or cheese if desired.