Cut the pomegranate into wedges and separate the seeds.
Reserve ¾ cup seeds for the salad.
In a mortar or mini-processor, mash enough of the remaining seeds to make 2 tablespoons of juice for the dressing.
Prepare the dressing: In a small bowl, whisk together the reserved pomegranate juice and the dressing ingredients: olive oil, sherry vinegar or red wine vinegar, and salt and pepper to taste.
In a large bowl, place the mixed salad greens, red onion, and scallions.
Toss the greens and pour the dressing over the salad, tossing again to coat evenly.
Scatter the reserved pomegranate seeds over the salad just before serving.