Heat a large Dutch oven or 5–7 quart pot over medium-high heat and add 1 tablespoon olive oil.
Add 1 pound lean ground turkey, spread it across the bottom of the pot, 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 3 minced garlic cloves and 1/2 cup diced onion to the browned turkey and cook 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 8 ounces spaghetti in half and add to the pot in small handfuls, spreading the pasta and alternating directions so it lies evenly; push the pasta gently under the liquid but do not stir yet.
Bring the pot to a boil, then cover and reduce heat to medium to maintain a simmer. Cook covered for 13–15 minutes, stirring every 5 minutes to prevent clumping, until the pasta is tender.
When the pasta is tender, remove from heat and let stand, covered, for 2–3 minutes to thicken slightly, then serve.
