1 To prepare marinade, combine balsamic vinegar, salt, canola oil, pepper and garlic in a large resealable plastic bag.2 Add asparagus and squeeze bag to coat asparagus with marinade; seal bag. Let stand for 30 minutes. 3 Remove asparagus from marinade. Discard marinade.4 Grill asparagus over medium heat on barbecue, turning occasionally, until tender, about 5-7 minutes.