Bring broth to a boil in a Dutch oven or soup pot. Stir in pasta. Cook, uncovered, over medium-high heat, stirring occasionally, until pasta is just tender, about 5 minutes. Stir in asparagus; cook for 2 minutes. Reduce heat to medium.
Break eggs into a large measuring cup and whisk until well blended. Add to the gently boiling soup in a thin, steady stream, stirring constantly with a fork. (Slow stirring will produce large threads; rapid stirring will break the threads up into small pieces.) Remove from heat and stir in lemon juice. Taste, adding salt if desired.