Prepare the asparagus: Peel the white asparagus carefully, starting from just below the tip. Cut off the woody ends and cut the spears into 1-inch pieces.

Sauté the aromatics: In a large pot, melt the butter over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.

Make the roux: Sprinkle the flour over the onions and stir well to combine. Cook for 1-2 minutes to remove the raw flour taste.

Add liquids: Gradually whisk in the vegetable broth, ensuring no lumps form. Bring to a gentle simmer.

Cook the asparagus: Add the asparagus pieces, lemon zest, and sugar to the pot. Simmer for 15-20 minutes until the asparagus is very tender.

Blend the soup: Use an immersion blender to puree the soup until completely smooth. Alternatively, carefully transfer to a blender in batches.

Finish the soup: Stir in the heavy cream and lemon juice. Season with salt and white pepper to taste. Heat through but do not boil.

Serve: Ladle into bowls and garnish with fresh chives. Serve with crusty bread if desired.