To prepare onions, combine first 3 ingredients in a small saucepan. Bring to a boil; remove from heat. Reserve 2 tablespoons vinegar mixture. Combine remaining vinegar mixture and onion in a small bowl; cool to room temperature.
To prepare dressing, combine reserved 2 tablespoons vinegar mixture, orange juice, oil, and mustard; stir well with a whisk.
Combine dressing and salad greens in a large bowl; toss well. Divide salad greens mixture evenly among 6 salad plates. Top each with 1/3 cup clementines, about 2 tablespoons onions, 2 tablespoons Spiced Walnuts, and 1 tablespoon pomegranate seeds.
Outstanding! Tangy vinaigrette compliments the citrus and the pomegranate seeds and nuts provide a contrasting texture and crunch. Worthy of serving to company or for a special occasion. Didn't have time to make the spiced walnuts, so used roasted pecans I had on hand. Will definitely try it with the spiced nuts next time.