Heat a large Dutch oven or 5–7 quart pot over medium-high heat and add the olive oil.
Add the ground turkey, spread it in an even layer, and season with the salt and pepper. Cook, stirring and breaking up the meat, until browned and cooked through, about 7–10 minutes.
Add the minced garlic and diced onion to the pot and cook with the turkey for about 1 minute, until fragrant and slightly softened.
Pour in the marinara sauce, low-sodium chicken broth, balsamic vinegar, Italian seasoning, and parsley; stir gently to combine.
Break the spaghetti in half and add it to the pot in small handfuls, spreading the pasta out and alternating directions so it lies submerged without clumping. Gently press the pasta 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 sticking, until the pasta is tender.
When the pasta is tender, turn off the heat and let the dish stand 2–3 minutes to thicken slightly, then serve topped with additional parsley or cheese if desired.