Place asparagus and onion in a saucepan with 1/2 cup vegetable broth. Bring the broth to a boil, reduce heat and let simmer until the vegetables are tender.
Reserve a few asparagus tips for garnish. Place remaining vegetable mixture in an electric blender and puree until smooth.
Melt butter in the pan that was used for simmering the asparagus and onions. Stir while sprinkling flour, salt, and pepper into the butter. Do not let the flour brown. Allow the mixture to cook only 2 minutes. Stir in remaining 1 1/4 cups vegetable broth and increase the heat. Continue stirring until the mixture comes to a boil.
Stir the vegetable puree and milk into the saucepan. Whisk yogurt into the mixture, followed by lemon juice. Stir until heated through, then ladle into bowls. Garnish with reserved asparagus tips. Sprinkle with Parmesan cheese if desired.