In a large mixing bowl, combine the lean ground beef, breadcrumbs, grated Parmesan cheese, egg, minced garlic, salt, black pepper, and chopped parsley. Mix gently with your hands until just combined. Be careful not to overmix, as this can make the meatballs tough.
Using a spoon or your hands, form the mixture into meatballs about 1 to 1.5 inches in diameter. Place them on a plate or baking sheet as you go.
Turn your Instant Pot to the "Sauté" mode. Add a drizzle of oil to the pot, then carefully place the meatballs in the pot in a single layer. Sauté for about 5 minutes, turning occasionally, until they are browned on all sides. You may need to do this in batches to avoid overcrowding the pot.
Once the meatballs are browned, remove them from the pot and set them aside. Pour in the beef broth, scraping the bottom of the pot to deglaze and release any flavorful bits stuck to the bottom. Add the marinara sauce and stir to combine.
Carefully place the browned meatballs back into the pot, ensuring they are submerged in the sauce. Close the lid and set the valve to "Sealing." Select the "Manual" or "Pressure Cook" setting and cook on high pressure for 10 minutes.
Once the cooking time is complete, allow the pressure to release naturally for 5 minutes, then do a quick release to release any remaining pressure. Carefully open the lid, and your Instant-Pot Meatballs are ready to serve!