Place asparagus and onion in a saucepan with 1/2 cup vegetable broth. Bring the broth to a boil, drop heat and allow simmer until the vegetables are tender.

Reserve a few asparagus tips for garnish. Place remaining vegetable mixture in an electric blender and purée until smooth.

Melt butter in the pan that was used for frying the asparagus and onions. Stir while sprinkling flour, salt, and pepper into the butter. Do not allow the flour brown. Allow the mixture to simmer only 2 minutes. Stir in remaining 1 1/4 cups veggie broth and increase the heat. Continue stirring until the mixture comes to a boil.

Stir the vegetable puree and milk into the pot. Whisk yogurt into the mixture, followed by lemon juice. Stir until heated through, then ladle into dishes. Garnish with reserved asparagus tips. Sprinkle with Parmesan cheese if desired.