In a medium mixing bowl, combine olive oil, balsamic vinegar, garlic powder, salt, pepper, and optional chili flakes. Whisk until smooth.

Chop bell peppers, zucchini, and red onion into uniform pieces, approximately 1-inch chunks.

Toss chopped veggies with the marinade in a bowl until well coated. Allow to marinate for at least 15 minutes.

Preheat the grill to medium-high heat (400-450°F).

Grill the marinated vegetables for 8-10 minutes, turning occasionally until tender and slightly charred.

Remove from grill and serve as a side dish or incorporate into salads, wraps, or grain bowls.