In a mixing bowl, combine the lean ground beef, Italian bread crumbs, finely minced onion, grated Parmesan cheese, minced garlic, salt, and ground black pepper. Mix gently until all ingredients are well incorporated, being careful not to overwork the meat.
Shape the mixture into meatballs, about 1-1.5 inches in diameter. You should have around 12-15 meatballs depending on size.
Set your Instant Pot to the sauté function and add 2 tablespoons of olive oil. Once the oil is hot, carefully add the meatballs in batches, browning them on all sides for about 2-3 minutes. Remove the meatballs and set them aside.
With the Instant Pot still on sauté, add the beef broth to deglaze the bottom of the pot. Scrape up any brown bits to prevent burning during pressure cooking.
Layer the broken spaghetti in the pot, followed by the jar of pasta sauce and the can of petite diced tomatoes. Do not stir; instead, gently press the pasta down to submerge it in the liquid.
Place the browned meatballs on top of the pasta and sauce mixture. Close the Instant Pot lid and ensure the steam release valve is set to sealing.
Set the Instant Pot to cook on high pressure for 8 minutes. Once the timer goes off, allow for a natural release for 5 minutes, then switch the valve to venting to release any remaining pressure.
Carefully open the lid and fluff the spaghetti with a fork. Serve hot, garnished with additional grated Parmesan cheese if desired.