Creamy Yogurt Asparagus Soup

Asparagus is one of my husband's favorite veggies.  I, however, prefer it drenched with butter, dressing, and just about anything else that will hide the asparagusiness of it all.  Maybe a lovely soup will make it better?  The soy and yogurt should give it a little zip and hopefully I can manage the flour bit without burning it to a brown goo.

Here goes:

• 1 pound fresh asparagus
• 3/4 cup chopped onion
• 2 cups vegetable or chicken broth
• 1 tablespoon butter
• 1 thick large leek, white stalk only
• 2 tablespoons all-purpose flour
• 1 teaspoon salt
• 1 pinch ground black pepper
• 1 cup soy milk
• 1/2 cup yogurt (or soy yogurt)
• 1 teaspoon lemon juice
• 1/4 cup grated Parmesan cheese (replace with sprinkle of garlic croutons or bread crumbs if you'd prefer to skip the dairy)

1. Place asparagus, leek and onion in a saucepan with 1/2 cup vegetable broth. Bring the broth to a boil, reduce heat and let simmer until the vegetables are tender.
2. Reserve a few asparagus tips for garnish. Place remaining vegetable mixture in an electric blender (or hand-held blender) and puree until smooth.
3. 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 broth and increase the heat. Continue stirring until the mixture comes to a boil.
4. Stir the vegetable puree and milk into the saucepan. Whisk yogurt 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. 

As always, I've included vegan/dairy-free options. I'll miss the Parmesan, let me know if you have any ideas for other tasty replacements.