1. Heat butter and oil in medium saucepan over medium-high heat. Add fennel and onion and cook 5 minutes or until beginning to soften, stirring occasionally. Add broth and bring to simmer. Reduce heat to medium-low, cover and simmer 20 minutes or until fennel is very tender. Stir in watercress and cook 30 seconds or just until wilted.
2. Puree soup in blender until very smooth. Pour soup back into pot and warm over low heat. Mix in lemon juice. Season with salt and pepper, if desired.
3. Ladle soup into a bowl and top with a 1 Tbsp dollop of yogurt, and garnish with fennel fronds and or watercress sprigs.