Using medium-high heat, warm the oil in a sauté pan and add the onion. Sauté 2 minutes then add the garlic, sautéing 1 minute. Add shrimp and cook 2 minutes additional.
Add the butter through parsley and stir until well mixed. Bring sauce to just below a boil. Take one ladle full of sauce without any shrimp and mix with just drained pasta in pasta pan. Plate pasta and then ladle shrimp and more sauce over plated pasta.