Heat oil in pressure cooker set to Sauté on Normal. Sauté onion in hot oil until fragrant, about 2 minutes; add tomatoes and garlic and continue to sauté until tomatoes start to break down, about 4 minutes. Pour chicken stock into the pot; add julienned basil, salt, pepper and red chili flakes; stir. Push spaghetti into the mixture with a spoon to cover in the tomato mixture. Lock pressure cooker lid in place. Set steam vent to Sealing. Select Pressure Cook (Manual) and cook for 5 minutes on High pressure. Press Cancel. Allow pressure to release naturally for 5 minutes, then set steam vent to Venting to quick-release remaining pressure. Stir mixture to break up any clumps of pasta. Spoon pasta into bowls; top with Parmesan cheese and basil to serve.