Cut zucchini in half lengthwise and again lengthwise into quarters. Cut into 1” pieces. Cut peppers in half lengthwise and remove stems, ribs, and seeds. Peel onion and trim root end slightly. Cut into 8 wedges, leaving root intact.

Add 2 Tbsp. olive oil, garlic, and oregano to a large bowl and stir to combine. Add zucchini, peppers, and onion wedges. Season with salt and pepper and toss to coat.

Heat a Grill Frying Pan on grill over medium-high until hot, 3-5 minutes.

Add vegetables to the Grill Frying Pan and cook, stirring occasionally, until well-charred, 7–10 minutes.

Transfer vegetables to a Serving Platter. Drizzle with vinegar and remaining 1 Tbsp. olive oil. Taste and season with additional salt and pepper, if desired. Top with feta, parsley, and sprinkle sumac over top.