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