Preheat oven to 200ºC (180º fan-forced). Place sweet potato and brussels sprouts in a large baking tray, drizzle with half the oil and season. Toss to coat. Cook for 25-30 minutes or until golden and tender, adding pancetta for last 10 minutes.
Meanwhile, heat a barbecue or char-grill pan over high heat. In a small bowl combine remaining oil and harissa paste. Brush steaks with marinade and season. Cook steaks for 4 minutes each side or until cooked to your liking. Set aside on a plate loosely covered with foil for 5 minutes.
In a screw top jar combine buttermilk, garlic, zest and chives. Season and shake well to combine.
Slice steak across the grain. Serve with roast vegetables and iceberg. Drizzle iceberg with dressing and sprinkle with extra chives.
Rump, sirloin or eye fillet would also work well in this recipe.
Harissa paste is available at most supermarkets, green grocers and delicatessens.
Asparagus, broccolini and pumpkin would also work well for the roasted vegetables.