In a medium sauce pan, bring 3/4 cup chicken stock to a simmer and add the asparagus pieces and chopped onion. Continue to cook until vegetables are just cooked through. Puree the mixture in an electric mixer until smooth.

In the same pan that was used for simmering the asparagus and onions, melt the butter. 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 chicken stock 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 the sour cream and lemon juice. Stir until heated through, then ladle into bowls. Garnish with blanched asparagus tips and sprinkle with Parmesan cheese.

Serve with just toasted crostini or baked pita chips.