On a clean cutting board, place the Aussie lamb rib roast and use a sharp knife to divide it into individual single-bone sections. Place the lamb into a large bowl. Add 1 1/2 tablespoons of the oil, 1 1/2 tablespoons garlic pepper, and 1/2 tablespoon Old Bay seasoning. Toss until the lamb is evenly coated.

Place the potatoes in a separate large bowl. Add the remaining 2 1/2 tablespoons of oil, 1/2 tablespoon of garlic pepper, and the remaining 1/2 tablespoon of Old Bay seasoning. Toss to coat.

Spread the potatoes in a single layer on a large sheet pan. Place the sheet pan into the oven and roast for 10 minutes.

While the potatoes cook, place the broccoli, onion, zucchini, and peppers into the same bowl used for the potatoes and toss to coat in any remaining seasoning and oil.

Remove the sheet pan from the oven. Add the remaining vegetables and the seasoned lamb to the sheet pan, arranging everything in a single layer so the vegetables roast rather than steam. Because sheet pan dinners should come out looking like dinner, not a crowded parking lot.

Return the sheet pan to the oven and roast for 35–40 minutes, or until the lamb reaches your preferred doneness and the vegetables are tender and lightly caramelized.

Let the lamb rest for 5 minutes, then serve.