In a soup pot melt butter and add onion, celery, leek, salt, and pepper. Cook vegetables for 5 minutes until translucent. Add curry powder and cook 2 minutes. In cheesecloth, combine bay leaf and thyme and tie to form an herb sachet. Add sachet of herbs to vegetables. Have stock simmering in saucepan. Add hot stock to vegetables, boil, reduce heat, and simmer for 20 minutes. Add peas and simmer for additional 10 minutes.
Remove sachet of herbs and discard. With immersion blender, puree soup (or puree with care in batches in a conventional standing blender). Return soup and pot to heat, add creme fraiche, being careful not to allow soup to boil, reheat. Add mint and serve.