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.