Supreme each grapefruit by slicing off the top and bottom ends. With a sharp knife, slice off the entire peel. Carefully cut each segment free by slicing between the segment and the membrane on each side. Do this over a bowl to catch any juices.

Whisk 1/4 cup of the collected juices with the walnut oil, mustard, garlic, and salt and pepper to taste, until well-combined.

Place the watercress in a large bowl, and toss with half the dressing. Transfer to a serving dish, and top with the grapefruit segments and kiwi slices. Drizzle with the remaining dressing. Garnish with the Pecorino, and serve.