In a large resealable plastic bag, mix the olive oil, parsley, oregano, basil, vinegar, kosher salt, pepper, and garlic. Place the onion, asparagus, mushrooms, eggplant, red bell pepper, and yellow bell pepper into the bag. Seal, and marinate 2 hours in the refrigerator, turning occasionally
Preheat the grill for high heat.
Lightly oil the grill grate. Grill the vegetables 6 minutes on each side, until tender.