1. To prepare dressing, combine all ingredients in a blender and process until smooth.
2. To assemble the salad, tear the romaine leaves off the root end and place romaine leaves on each plate. Scatter the lamb slices, red onions, and preserved lemon slices on the salad. Drizzle with dressing as needed. Do the leftover skhug, if using. Sprinkle with feta and pistachios.