Wash asparagus, gently bend one stalk and let it snap naturally- throw away the snapped end. Use that as a guide and snap or chop the ends off all the asparagus to avoid stringy soup. Boil 1/2 cup of the chicken broth and steam asparagus until tender. Reserve a few asparagus for garnish. Transfer remaining asparagus and liquid into blender and puree.
In same saucepan melt butter. Add flour and slowly whisk in remaining chicken broth until thickened. Stir in salt, pepper, asparagus puree and milk. Add yogurt and lemon juice and stir until heated through. Top with parmesan cheese and garnish with asparagus spears.