How to Make It
Preheat the oven to 350°. In a small saucepan, combine the walnuts, sugar and water; bring to a boil, stirring, until the sugar dissolves. Cook over moderate heat, stirring, for 5 minutes. Using a slotted spoon, transfer the walnuts to a pie plate and sprinkle lightly with salt. Bake for 7 minutes, or until the nuts are fragrant and lightly caramelized. Let cool.
In a small bowl, whisk the vinegar, orange juice, grapeseed oil, olive oil and sesame oil; season with salt and pepper.
In a large bowl, toss the spinach with the watercress, plums, apple and Gruyère. Add the dressing, season with salt and pepper and toss. Scatter the walnuts on top and serve.