Preheat the salt slab on the grill to 400F.
Butterfly pork by cutting through the pork lengthwise, making sure to leave about 1 in of pork at the bottom uncut, then pound out pork to ½” thickness. Season pork with olive oil, rub, salt, and pepper. Let sit for 15 minutes or up to 24 hours.
Meanwhile, in a food processor pulse spinach, cheese, red pepper, onion, egg yolk, and panko until well combined.
Spread the filling into the pork evenly, leaving a 1 inch border on all sides. Starting on the long side, roll up pork and secure with toothpicks. Top with smoked balsamic.
Place the pork tenderloin on the grill. Close the lid and cook for 15 minutes or until pork is no longer pink.
Let rest 10 minutes before cutting. Finish with a drizzle more of balsamic.
