Heat oil in nonstick skillet, over medium-high heat. Add onions and garlic; cook, stirring, for about 8 minutes or until starting to turn golden. Reduce heat to medium-low and cook, stirring occasionally, for about 10 minutes or until softened. Add balsamic vinegar and cook for 2 minutes. Remove from heat.
Add pepper, bread crumbs, egg yolk, basil, oregano, cheese and half each of the salt and pepper; stir to combine well. Set aside.
Using a sharp chef's knife, start cutting pork loin lengthwise in a spiral fashion to "unroll" the loin until you end up with 1 long rectangular piece. Using a meat mallet, pound out pork to an even thickness. Spread with mustard and sprinkle with remaining salt and pepper. Spread onion mixture over top leaving 1-inch (2.5-cm) at 1 short end. From other end start rolling up pork loin, jelly-roll fashion.
Using kitchen string, tie the pork loin to secure at 1-inch (2.5-cm) intervals. Place on large plate; set aside.
Heat both sides of barbecue at high heat. Turn off 1 side of barbecue and reduce heat of other burner to medium. Place pork loin on greased grill over unlit side of barbecue. Close lid and cook for 45 minutes, adjusting heat to keep temperature of barbecue at about 325ºF (160ºC).
Turn pork roast and cook for another 45 minutes, maintaining temperature. Turn roast again and cook for about 30 minutes or until meat barbecue thermometer inserted in centre registers 155ºF (68ºC).
Transfer to cutting board and let stand covered with foil for about 10 minutes. Remove string and cut into thick slices.