Set a pressure cooker to sauté. Add olive oil and onion; cook until softened.
Add ground beef, Italian seasoning, garlic powder, salt, pepper, and minced garlic. Cook until beef is browned.
Add water and scrape the bottom of the pot well to prevent sticking.
Break spaghetti in half and arrange it in a crisscross layer. Pour spaghetti sauce and diced tomatoes over the pasta.
Pour remaining water around the sides and press pasta down gently so it is mostly covered.
Lock the lid, set valve to sealing, and cook on high pressure for 5 minutes. Let pressure release naturally for 5 minutes, then quick-release remaining pressure.
Toss pasta, sauce, and meat together. Serve with a salad and garlic bread if desired.