1) For the salad: Put the spinach in a salad bowl and arrange the orange segments and walnuts decoratively on top.
2) For the vinaigrette: In a small clean bowl, using a wire whisk, mix the vinegar, sugar and thyme. Drizzle in the oil, whisking constantly. Season with salt and pepper, to taste.
3) Drizzle the thyme vinaigrette over the salad and serve.