2largesweet onions, peeled and cut horizontally into 1/2-inch-thick slices
Add onion slices to shallow dish.
In a medium bowl, combine olive oil, Earth Balance butter, Dijon mustard, agave, thyme, and Worcestershire sauce. Pour over onion slices and refrigerate for at least 30 minutes.
Preheat grill to medium-high heat.
Add onion slices to thin metal skewers and brush both sides of onion slices with oil mixture. Place onions on grill and cook until beginning to char, turning and brushing often with remaining oil mixture, about 15-20 minutes.