Bring a large pot of salted water to a boil. Add noodles; bring back to a boil and cook, stirring often, until tender, 5 to 7 minutes.
Melt butter in a small saucepan over medium-high heat. Add onion and cook, stirring, until softened but not browned, 3 to 5 minutes. Stir in parsley and remove from heat.
Drain noodles, reserving 1/4 cup cooking liquid. Return noodles to pot and stir in butter-onion mixture. Season with salt and pepper. If noodles appear dry, add reserved cooking liquid 1 Tbsp. at a time. Serve hot.