- Prepare grill over medium-high heat. Thirty minutes prior to grilling, equally divide onions, squash, zucchini, and garlic into two large plastic storage bags.
- Whisk together oil, vinegar, salt, and pepper until thoroughly combined. Evenly divide marinade and pour over vegetables. Using your hands, toss ingredients together, being careful to keep the onion rings intact (if you have kept the onions in rings). Marinate for 30 minutes.
- Grill vegetables, either setting directly over heat or threaded onto skewers. Note: onions will be 3-4 minutes per side, while squash and zucchini will be 2-3 minutes per side. Remove and serve.