Cut off and discard the woody end of 1½ pounds asparagus (about about 1 inch or 2.5 cm). Cut the spears into discs and leave the tips whole.In a large skillet, sauté 1 small onion (chopped) in 2 tablespoons extra virgin olive oil for 3 minutes.Add asparagus, 3 cloves garlic (grated), ¾ teaspoon salt, and ⅛ teaspoon black pepper, and cook for 5 minutes or until the asparagus are bright green and fork-tender. Add 1 cup of water after 1 minute.
To a food processor, add half of the cooked asparagus, ½ cup fresh basil, ½ cup grated parmesan, the juice of ½ lemon, 2 tablespoons of water, and blend until creamy.Cook the pasta in plenty of salted boiling water as per package instructions.
Add the sauce back to the skillet with the remaining asparagus, add the pasta, and 1/2 cup of reserved pasta water.Stir until creamy adding more reserve cooking water and parmesan if necessary.Taste and adjust for salt before serving with a drizzle of extra virgin olive oil and a grating of lemon zest.