Add zest and juice of lemons to skillet. Stir and let simmer about a minute.
Add drained spaghetti to skillet, tossing to coat. Gradually add reserved pasta water to create glossy sauce.
Stir in chopped parsley. Season with salt and black pepper to taste.
Sprinkle Parmesan over pasta and toss until melted and incorporated, if using.
Serve immediately with extra parsley and Parmesan, if desired.
Notes
Leftovers will keep for 3-4 days in the fridge. Reheat over low heat and stir to avoid drying out. For freezing, coat with a bit of olive oil and freeze for up to 1 month.