Preheat the oven to 200°C, fan 180°C, gas 6. Place the halved tomatoes, garlic and red onion in a roasting tray and season with freshly ground black pepper. Drizzle with the olive oil and roast for 30 minutes.
Transfer the roasted tomatoes, onions and garlic to a pan, along with the diced potatoes and the vegetable stock. Bring to the boil, the squeeze the garlic cloves out of the skins and add to the pan with the torn basil leaves. Simmer for 12-15 minutes, until the potatoes are tender.
Meanwhile, 8-10 minutes before the end of the soup cooking time, place the part-cooked baguette on a tray and bake in the oven 8-10 minutes.
Blend the soup to a smooth consistency using a hand blender. Ladle into 4 bowls and garnished with the extra torn basil leaves and a drizzle of olive oil. Serve with slices of warm baguette.