Step #1 Mix together the first 3 ingredients in a shallow dish or a heavy-duty zip-lock plastic bag.
Step #2 Add in the tenderloins; cover dish or seal bag; chill for 2 hrs & turn meat every once in awhile.
Step #3 Remove tenderloins from marinade & throw out marinade.
Step #4 Grill tenderloins, covered with grill lid, over high heat for 6 to 7 1/2 mins on each side or until a meat thermometer registers 160°.
Step #5 To make Onion-Balsamic Sauce: cook onion In a large-ish skillet that has been sprayed with cooking spray over med-heat/flame for about 15 mins, stirring frequently.
Step #6 Stir in sugar, stirring every once in awhile, cover & cook, for 10 mins.
Step #7 Add in wine, vinegar, & bouillon cube.
Step #8 Cook, uncovered, stirring frequently, until liquid is reduced by half.