Prepare Asparagus: Start by trimming the woody ends off the asparagus spears, then cut the tips about 1 1/2 inches from the top and set aside. Slice the remaining stalks into uniform 1/4-inch rounds, ensuring even cooking and a smooth texture when puréed. Sauté Aromatics: In a 4–5 quart stockpot, melt the butter over medium heat until shimmering. Add the chopped onions and cook until translucent, soft, and fragrant, about 5 minutes, then stir in the minced garlic and cook for an additional minute to release its aroma. Cook Asparagus Stalks: Add the chopped asparagus stalks to the pot, sprinkle with salt and pepper, and sauté for roughly 5 minutes, stirring frequently to ensure the vegetables are lightly tender and infused with the butter-onion-garlic mixture.
Simmer with Broth and Herbs: Pour in the chicken or vegetable broth and water, then add the fresh thyme sprigs and bay leaf. Bring the mixture to a gentle boil, reduce heat to a simmer, cover, and cook until the asparagus is tender, about 10 minutes, allowing flavors to meld. Add Fresh Parsley: Once the asparagus is tender, remove from heat momentarily and stir in the chopped fresh parsley to introduce a bright, herbaceous note and enhance the overall flavor profile of the soup.
Blanch Asparagus Tips: While the soup simmers, bring a small pot of salted water to a boil and blanch the reserved asparagus tips for 2–4 minutes until just tender, then drain and rinse under cool water to stop cooking and preserve the vibrant green color.
Purée Soup: Remove the thyme sprigs and bay leaf from the pot, then purée the soup using an immersion blender directly in the pot or in batches with an upright blender. For an extra-smooth texture, press the puréed soup through a fine-mesh sieve. Enrich and Season: Stir in the heavy cream or full-fat sour cream for creaminess, followed by the dry vermouth and lemon juice for depth and brightness. Taste and adjust seasoning with salt and black pepper as needed. Serve and Garnish: Ladle the soup into bowls and carefully place the blanched asparagus tips on top as a garnish, adding a pop of color and texture that makes each serving visually appealing and restaurant-quality.
