In a medium saucepan stir together water, bouillon granules, fresh lemongrass or lemon peel, fresh cilantro, and white pepper. Bring to boiling; reduce heat to low. Simmer, covered, for 15 minutes. Strain the liquid, discarding the solids; return the liquid to the saucepan.
Return liquid to boiling. Set aside a few of the asparagus tips for garnish. Add the remaining asparagus to saucepan. Reduce heat to low. Simmer, uncovered, for 8 to 10 minutes or until asparagus is tender. Remove from heat; cool slightly.
Carefully transfer the asparagus-broth mixture to a blender container. Cover and blend until smooth; set aside.
In the same saucepan gradually stir the evaporated milk into the cornstarch. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 2 minutes more. (The mixture may be slightly foamy.) Gradually add the asparagus mixture, stirring constantly. Heat through.
To serve, ladle the asparagus soup into soup bowls. If desired, swirl a little dairy sour cream into each serving and top with the reserved asparagus tips.