Preheat the oven to 200°C/Fan 180°C/Gas 6. Prick the potatoes all over with a fork, put on a baking tray and mist with cooking spray. Bake for 1 hour 20 minutes, until tender with a crisp, golden skin.
Meanwhile, put the tomatoes and thyme in a small baking dish and drizzle over the oil and balsamic vinegar. Season to taste and toss to coat. Bake alongside the potatoes for the final 35 minutes of cooking time.
Remove the potatoes and tomatoes from the oven, and discard the thyme. Split the potatoes and top with the ricotta and roasted tomatoes. Drizzle over any juices from the baking dish, season with freshly ground black pepper and serve.