Preheat the oven to 220°C. Combine the oil, vinegar, basil and brown sugar in a small bowl. Cut the tomatoes in half and place in an ovenproof dish just large enough to fit them in one layer. Sprinkle with the oil mixture.
Spray a small baking dish with cooking spray. Coat the roasts with the pesto and place in the baking dish along with about 1/2 cup water. Roast the lamb for 15 to 18 minutes for rare (the lamb will be pink), 20 to 25 minutes for medium, or 30 to 35 minutes for well done, basting occasionally with the pan juices. You can add more water to the pan if the juices evaporate too quickly. Roast the tomatoes at the same time.
Remove the lamb roasts from the oven and cover with foil. Rest for 5 to 10 minutes to let the juices settle before carving. Serve with the oven-roasted tomatoes along with the cooked fusilli, if desired.