Step 1: Prepare the asparagus. Snap off the woody ends of the asparagus spears. Cut the tender parts into 1-inch pieces. If desired, reserve a few asparagus tips for garnish.
Step 2: Sauté aromatics. In a large pot or Dutch oven, melt the butter (or heat olive oil) over medium heat. Add the chopped onion and cook until softened and translucent, about 5-7 minutes.
Step 3: Add garlic. Stir in the minced garlic and cook for another minute until fragrant, being careful not to burn it.
Step 4: Cook asparagus. Add the chopped asparagus pieces (excluding reserved tips) to the pot. Sauté for 3-5 minutes, allowing them to brighten in color and become slightly tender-crisp.
Step 5: Simmer the soup. Pour in the vegetable or chicken broth. Bring the mixture to a gentle boil, then reduce the heat, cover, and simmer for 10-15 minutes, or until the asparagus is very tender.
Step 6: Blend until smooth. Carefully transfer the hot soup mixture to a blender (or use an immersion blender directly in the pot). Blend until completely smooth and creamy. If using a standard blender, work in batches and vent the lid to release steam.
Step 7: Finish and season. Return the blended soup to the pot over low heat. Stir in the heavy cream (or chosen dairy/non-dairy alternative). Season generously with salt and freshly ground black pepper to taste. Add the fresh lemon juice, if using, to brighten the flavors.
Step 8: Garnish and serve. If you reserved asparagus tips, quickly blanch them in boiling water for 1-2 minutes until tender-crisp, then add to the soup or use as a garnish. Serve hot, optionally topped with fresh chives or dill.