Preheat grill to medium-high. Melt butter in a saucepan over medium-high heat. Add onion and garlic; sauté 4 minutes, stirring constantly. Stir in brown sugar, vinegar, ketchup, and soy sauce; bring to a boil. Cook 6 minutes. Process mixture until smooth. Reserve 6 tablespoons sauce. Sprinkle pork tenderloins with salt; brush with sauce. Place pork on an oiled grill rack. Grill 6 minutes on each side or until thermometer registers 145°. Remove pork, and let stand for 5 minutes. Slice.