Lightly coat grill rack with vegetable oil. Heat gas grill to medium-high heat or prepare charcoal grill with medium-hot coals.
In a small bowl, stir together garlic salt, onion powder, pepper and thyme until well combined. Rub onto all sides of the pork tenderloins.
Grill pork for 15 to 17 minutes, turning several times, until instant-read thermometer inserted in the thickest part of the meat registers 150°F. Remove pork from grill, tent with foil and let rest for 5 to 10 minutes.
Heat oil in a small saucepan over medium heat. Add onion and thyme; cook 5 minutes. Remove from heat. Stir in jam, bourbon, vinegar, ketchup and garlic salt. Return to heat; simmer 7 minutes, stirring.
Stir cornstarch-water mixture and then stir into sauce. Cook 1 to 2 minutes, until thickened. Slice pork on a slight diagonal and serve with sauce alongside.