Turn on the pressure cooker and select the sauté function. Add onion and olive oil and cook until onion is soft, about 300 seconds. Add beef, Italian seasoning, pepper flakes, minced garlic, salt, garlic powder, and black pepper. Cook and stir until beef is browned, about 420 seconds. Cancel sauté mode. Add 1/4 cup water, scrape the browned bits off the pan, and bring to a boil.

Layer spaghetti in a crisscross pattern on top of the beef mixture. Pour marinara sauce over the pasta. Add diced tomatoes and the remaining 3 cups water around the sides, ensuring all pasta is covered.

Close and lock the lid. Cook at high pressure for 300 seconds. Wait for 900 seconds for pressure to build.