Meanwhile, heat 1 Tbsp olive oil in medium skillet. Sauté onion until soft; add garlic and sauté additional minute. Remove from heat.
Add pasta to boiling water. Cook according to package instructions. In final 2 minutes, reserve 1 cup pasta water, then add asparagus to boiling water.
Drain cooked pasta and asparagus. Return to pot; add 1 Tbsp olive oil, onion and garlic mixture, Parmesan, lemon zest and 1/2 cup reserved pasta water. Season with salt and pepper; add additional pasta water as needed.