Stir together first 6 ingredients; remove half of mixture, and chill.
Place tenderloins in a shallow dish or heavy-duty zip-top plastic bag; pour remaining molasses mixture over tenderloins. Cover or seal, and chill 8 hours, turning occasionally.
Remove tenderloins from marinade, discarding marinade.
Grill tenderloins, covered with grill lid, over medium heat (300° to 350°) about 25 minutes or until a meat thermometer inserted into thickest portion registers 160°, turning occasionally.
Sauté onion and garlic in 1 tablespoon hot oil in a saucepan until tender. Add wine and broth; cook over medium-high heat, stirring occasionally, until mixture is reduced by three-fourths. Stir in remaining molasses mixture, and simmer 5 minutes. Pour through a wire-mesh strainer, discarding onion and garlic; return to pan.
Stir together cornstarch and water. Stir into wine mixture.
Bring to a boil over medium heat, stirring constantly. Boil, stirring constantly, 1 minute.
Stir in 1/2 teaspoon salt. Serve with sliced tenderloin. Garnish, if desired.