1. Peel 3 grapefruits and the oranges, removing all the pith. Slice them into rounds. Arrange them on serving plates. Scatter the lamb's lettuce, walnuts, pumpkin seeds, and pistachios over the top.
2. Prepare the vinaigrette. In a small bowl, whisk together the olive oil, lime juice, a pinch of salt, and pepper. Drizzle over the salad just before serving.