Place lamb on a baking tray, brush with 1 tablespoon oil and sprinkle with chilli powder and rub in. Scatter oregano leaves over lamb and season with salt and pepper. Allow to come up to room temperature. Roast lamb for 20 minutes for medium-medium rare. Rest for 10 minutes before slicing.
Place sweet potato wedges in a bowl with 1 tablespoon oil and toss to coat. Place on a baking tray and roast for 30-40 minutes until crisp and golden.
Preheat a char grill over high heat and cook corn cobs on all sides until cooked with some grill marks. Lightly brush with oil, sprinkle with a little chilli powder and a generous grating of parmesan.
Serve well rested lamb, sliced into individual cutlets with corn, sweet potato wedges and extra oregano leaves.
Cooking the racks of lamb as 2 x 4-5 point racks means the lamb cooks more evenly. You can cook it as one large 8-10 point rack if you have some guests who prefer medium rare (in the middle section) and some who prefer medium (on the outer edges).
If you cook the lamb as one large rack, cook for 20-25 minutes and check the centre. You can also slice off the end pieces and return the middle to the oven for another few minutes.
You can place the corn cobs under a hot grill to melt the parmesan if you like.