Set Instant Pot to Sauté. Warm the olive oil until shimmering.
Sauté the onion until translucent, about 3 minutes. Stir in garlic and cook 30 seconds.
Add ground beef. Cook, breaking it up, until browned. Drain excess fat if needed.
Season with Italian seasoning, salt, pepper, and red pepper flakes. Splash in a bit of broth and scrape up browned bits.
Fan the spaghetti over the meat in crisscross layers. Do not stir.
Pour marinara evenly over the pasta. Add the remaining broth around the edges. Press noodles just under the liquid.
Lock the lid and set to High Pressure for 7 minutes.
Let pressure release naturally for 5 minutes, then quick-release the rest. Open and stir gently to separate noodles.
Rest 2–3 minutes to thicken. Stir in Parmesan and parsley. Taste and adjust seasoning, then serve.