Place asparagus and onion in a saucepan with ½ cup vegetable stock, bring 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 a blender and purée 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 cup vegetable stock and increase the heat. Continue stirring with a whisk until the mixture comes to a boil.

Stir the vegetable purée and milk into the saucepan slowly. Whisk yogurt into the mixture, followed by lemon juice and stir until heated through, then ladle into bowls.