Layer 3 sheets of foil overlapping to keep juices in pouch, place pork loin in center; fat side down.
Combine liquid smoke, garlic, onion, and celery salt, pepper, celery salts in bowl; mix to combine.
Rub spice mixture on all sides of pork loin.
Fold foil into a tent by rolling the sides together and down, then fold edges in.
Place on plate and allow to marinate in fridge for 2-3 hours.
When ready to cook, preheat grill on high.
Set foil wrapped pork loin in the center of grill and cook 22 minutes or until center of loin reaches 130 degrees F.
Remove from foil, place directly on grill, 2 minutes on each side or until reaches 140-142 degrees F.
Set sliced onion on grill and cook 5-6 minutes; turning once.
Set on serving plate and allow to rest for 5 minutes.
Serve and enjoy our Grilled Southern Pork Loin Recipe!