To prepare the salad: Put the spinach in a salad bowl and arrange the orange segments and walnuts decoratively on top.
To prepare 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.
Drizzle the thyme vinaigrette over the salad and serve.