Slice the top and bottom off each orange, then remove the peel carefully and slice the segments. Save the juice for salad dressing. In a salad tray mix together spinach leaves, orange segments and scatter the chopped onions over the salad. To make the dressing, whisk together oil, orange and lemon juice, honey, mustard, salt and pepper in a bowl. Pour the dressing over the salad just before serving. Toss the salad well to coat the leaves with the dressing.