Preheat over to 200°C or 180°C fan-forced. Line 2 large baking trays with baking paper. Arrange sweet potato on 1 prepared tray and lightly spray with oil. Bake for 20minutes. Add onion to tray. Bake for 20 minutes or until vegetables are tender and lightly browned. Cool for 5 minutes.
Place tomatoes on remaining prepared tray and lightly spray with oil. Bake for 30 minutes or until tender and lightly browned.
Meanwhile, place sweet potato mixture into a large bowl. Add feta and rosemary and season with freshly grown black pepper. Toss to combine. Lightly spray a 20cm (base measurement) round ovenproof baking dish with oil. arrange sweet potato mixture over base of dish. Poor over egg, pressing vegetables down with a spatula to cover with egg. Bake in oven with tomatoes for 25-35 minutes or until golden and set.
Set frittata aside for 5 minutes. Cut into wedges and serve with roasted tomatoes.