Set the Instant Pot to Sauté and heat until hot.
Add the ground turkey or beef and the kosher salt; cook, breaking up the meat with a wooden spoon, about 3 minutes until mostly browned.
Add the diced onion and minced garlic and continue to sauté until the onion is softened, about 3–4 minutes.
Press Cancel. Pour in the 2 cups of water and use a wooden spoon to scrape up any browned bits from the bottom of the pot (this helps prevent a burn error).
Arrange the broken spaghetti over the meat in a crisscross or fanned layer, then pour the marinara sauce over the pasta so it is covered; do not stir.
Close and lock the lid. Cook on High Pressure for 9 minutes.
Quick-release the pressure by carefully turning the valve to venting, then open the lid. Stir the pasta and sauce to combine.
Serve topped with grated Parmesan if desired.