Place asparagus and onion in a large saucepan or stock pot with 1/2 cup vegetable broth.
Bring the broth to a boil, reduce heat and let simmer until the vegetables are tender (about 6-8 minutes).
Reserve 8 asparagus tips for garnish.
Pour vegetable mixture into an electric blender and purée until smooth.
Using the same saucepan melt butter over medium heat.
Slowly add flour, salt, and pepper while stirring continuously.
Cook for 2 minutes, being careful that the flour doesn't brown.
Stir in the remaining 2 1/2 cups vegetable broth and increase the heat to medium-high.
Continue stirring until the mixture comes to a boil.
Add the vegetable purée and milk to broth.
Whisk in 1 cup of yogurt and lemon juice.
Stir until heated through (careful not to boil to avoid curdling).
Ladle into bowls and garnish with yogurt and reserved asparagus tips.
Sprinkle with Parmesan cheese if desired.