1In a large pot boil 4 cups water with salt. When boiling, add asparagus and cook 3-5 minutes, until tender crisp. I steamed mine....make sure you save some of the water!!
Drain asparagus in colander reserving 1 cup liquid before draining.
Cook pasta. When water boils cook pasta according to package directions for al dente.
In a small bowl combine egg yolk, Parmigiano-Reggiano cheese, 1/4 cup reserved asparagus liquid, salt and pepper. Mix well. (Add the asparagus water a little at a time if it's till hot....you don't want to cook the egg yolk!!)
After pasta is drained return to pot and mix with egg mixture. Cook on medium-low about 2 minutes, until sauce thickens and sticks to pasta.
Toss in asparagus and mix well. Adjust salt and pepper to taste. If pasta seems too dry add more reserved liquid a tablespoon at a time.