Separate 1 dandelion leaf from the bunch; thinly slice, and set aside. Fill a large Dutch oven half full with water; bring to a boil.
Add remaining greens to boiling water; cover and cook for 2 minutes.
Drain.
Place greens in a food processor; process 30 seconds or until smooth.
Place oil and butter in a 3-quart saucepan. Cook over medium heat until butter melts.
Add onion; saut 3 minutes or until tender.
Sprinkle flour over onion mixture, stirring to coat.
Add broth and next 4 ingredients (through pepper), stirring with a whisk. Stir in pureed greens. Bring to a boil; reduce heat, and simmer, uncovered, 5 minutes, stirring occasionally.
Garnish servings evenly with reserved sliced greens. Top with sour cream, if desired.