Best Asparagus Soup

Bahan-bahan (3 porsi)

• 1 tablespoon butter
• 3/4 cup onion (chopped)
• 1 pound asparagus
• 1 tablespoon butter
• 2 tablespoons flour
• 1/2 teaspoon salt
• pinch pepper
• 1 & 3/4 cups chicken or vegetable broth
• 1 cup milk
• 1/2 cup plain yogurt (I used nonfat)
• 1 teaspoon lemon juice
• 1/4 cup Parmesan cheese (plus more for garnish)
• salt and pepper (to taste)