Cut the potatoes in half lengthways, then each half into 3 wedges. Put in a large roasting tin with all the remaining ingredients, season with salt and pepper and stir well to combine thoroughly.
Roast in a preheated oven, 200°C (400°F), Gas Mark 6, for 1 hour, or until the potatoes are cooked through. Stir the vegetables in the tin a couple of times during cooking. If the liquid dries out so much that the tomatoes and onions begin to stick to the base, add a little hot water to stop them burning.