1. Peel the top of the grapefruit. Set aside. 2. Cut the avocados in half and remove the central pit. 3. Using a round spoon, remove the avocado flesh in one piece. 4. Set the shells aside. 5. Cut the flesh into thick slices and place in a bowl. 6. Add the lemon zest, oil, vinegar, and grapefruit segments. 7. Season with salt and gently mix in the fennel. 8. Fill the avocado shells with this mixture and serve.