In a shallow dish or zip-lock bag, mix together balsamic vinegar, pressed garlic cloves, and diced onion.

Add the pork tenderloins to the marinade.

Cover the dish or seal the bag and chill for 2 hours, turning meat occasionally.

Remove tenderloins from the marinade and discard the marinade.

Grill tenderloins, covered with the grill lid, over high heat for 6 to 7.5 minutes on each side, or until a meat thermometer registers 160°F.

To make the Onion-Balsamic Sauce, spray a large skillet with cooking spray and cook the chopped purple onions over medium heat for about 15 minutes, stirring frequently.

Stir in the brown sugar, cover, and cook, stirring occasionally, for 10 minutes.

Add in the red wine, balsamic vinegar, and beef bouillon cube.

Cook, uncovered, stirring frequently, until the liquid is reduced by half.

Slice the tenderloins and serve with Onion-Balsamic sauce.