Place asparagus and onion in a saucepan with 1/2 cup of stock. Bring to a boil, reduce heat and let simmer until the vegetables are tender.
Reserve a few asparagus tips for garnish. Blend remaining vegetable mixture until smooth.
Melt butter in the saucepan 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 stock and increase the heat. Continue stirring until the mixture comes to a boil.
Stir vegetable puree and milk into the saucepan. Whisk yoghurt 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.
This is good but make sure you get rid of the tougher "woody" ends of the asparagus before you start cooking. They have this natural breaking point; if you hold the asparagus spear and bend it will break at the point where the asparagus starts to be a bit tough. - 07 Oct 2008