Melt butter in soup pot. Add onion, leeks, salt, and pepper and sweat for 5 minutes. In cheesecloth combine bay leaf and thyme and tie to form sachet. Add sachet of herbs to vegetables. Have stock simmering in saucepan. Add hot stock to vegetables, bring to the boil, reduce, and simmer for 20 minutes. Add asparagus bottoms, and continue to simmer for 10 minutes. Remove sachet of herbs. With immersion blender, puree soup (or puree in batches in a standing blender).
Carefully strain soup through fine sieve. Return strained soup to the pot and reboil. Add cream and heat through, but do not boil. Serve immediately with hot asparagus tips strewn in as garnish. Finish with a dollop of sour cream.
Please note that this soup may be chilled and served cold.