In a large saucepan, saute onion and garlic in oil until tender. Stir in the broth, parsley, carrot, basil, tarragon, salt, pepper and cayenne. Cut asparagus into 1-in. pieces; set tips aside. Add asparagus pieces to saucepan. Bring to a boil. Reduce heat; cover and simmer for 20 minutes or until vegetables are tender. Cool slightly., Puree soup in batches in a blender. Return to the saucepan. Stir in asparagus tips; cook for 5 minutes or until crisp-tender. Garnish with sour cream and tomatoes if desired.