In small bowl, combine reserved pineapple liquid, oil, vinegar, mustard and garlic; mix with wire whisk until well blended. Pour 1/4 cup of the pineapple liquid mixture into nonmetal dish or large resealable food storage plastic bag. Add pork tenderloin; turn to coat. Cover dish or seal bag. Refrigerate 1 to 4 hours to marinate.
Add mayonnaise to remaining pineapple liquid mixture; mix well. Cover; refrigerate until serving time.
Heat grill. When ready to grill, remove pork from marinade; discard marinade. Place pork on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium coals. Cook 15 to 20 minutes or until no longer pink in center, turning once. Cut pork diagonally into thin slices.
In large bowl, combine salad mix, pineapple tidbits and onions; mix well. Add mayonnaise mixture; toss to coat.
To serve, divide salad mixture onto 4 individual serving plates. Top each with pork slices.