1. Combine onion, mayonnaise, oil, vinegar, mustard and rosemary in a large bowl.
2. Coarsely chop the pork tenderloin and potatoes reserved from the “Grilled Pork & Potato Planks” recipe; stir into the mayonnaise mixture. Season with salt and pepper to taste.
3. Spoon equal amounts of salad on lettuce leaves to serve.