In small bowl, whisk together honey and vinegar. Add rhubarb and toss to coat. Let stand at least 5 minutes and up to 10 minutes, then add olive oil, 1/2 teaspoons salt and 2 teaspoons coarsely ground pepper. Meanwhile, cut away peel and white pith from oranges, then thinly slice.  In large bowl, toss spinach and watercress; fold in orange slices and divide among plates. Spoon rhubarb and dressing over each salad and top with pistachios and ricotta salata.