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; sauté 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. Serve with lemon wedges.