Heat the olive oil in a large pot. Sauté the asparagus over medium-high for 2-3 minutes, depending on thickness, or until crisp-tender. Remove to a plate.
Add the 6 cups of broth and the lemon zest. Increase the heat to high and bring the broth to a boil. Stir in the pasta and cook for 2 minutes. Reduce the heat to low.
In a medium bowl, beat the egg yolks. Slowly add one cup of the hot broth to the yolks, whisking as you add. Then whisk the egg mixture back into the soup pot. Whisking consistently as you pour the mixture back in will allow it to fully and smoothly incorporate into the broth.
Bring the soup back to a simmer for one minute, adding the sautéed asparagus, and salt and pepper to taste. I typically add about 1/2 teaspoon of kosher salt and 1/4 teaspoon of pepper. This may vary according to taste and what type of broth you use.
Remove soup from the heat, ladle into bowls, and garnish with the Parmesan cheese.