Place onion in a small bowl of cold water to soak. Set aside. Finely grate 1 tsp. zest and squeeze 2 tbsp. juice from lemon. Whisk the zest and juice, oil, salt and pepper in a large bowl. Peel grapefruit with a sharp knife, removing all peel and white pith.
Peel grapefruit with a sharp knife, removing all peel and white pith. Working over a medium bowl, cut the segments from the surrounding membranes and let them drop into the bowl. Squeeze the peels and membranes over bowl to extract all the juice before discarding them. Halve, peel and pitavocado. Cuth lenghtwise into 1/4 inch thick slices, then cut the slices in half. Gently stir the avocado and pomegranate seeds into the grapefruti mixture.
Finely slice basil and drain the onion; gently stir into rhe grapefruit mixture. Arrange lettuce leaves on 4 salad plates, top with the grapefruit mixture and serve immediately.